A blockchain is a decentralized database of transactions grouped into blocks. The blocks form chain where each block is cryptographically linked to the previous block. The decentralization of a blockchain means that copies of the chain are held in multiple different servers in different places. On the Steem blockchain, the servers and the people who run them are called witnesses. Steem is a so called Delegated Proof-of-Stake chain. What that means is that the owners of Steem Power get to vote for witnesses in a continual voting process. The blockchain rules are such that to have permission to create (top 20 witnesses) or confirm new blocks a witness account must have enough votes from all the other accounts all of which hold Steem Power. The votes are weighed according to the Steem Power they hold.
The community of Steem users is collectively responsible for voting the right people to run the witness servers. Their reliability and professionalism should be the key criteria. Because the witnesses approve of all the changes in the blockchain software, it is important that they know what they're doing. Usually, those who have a lot of stake (Steem Power) are knowledgeable and are able to judge who should be a witness. Their vote has the most weight. But all the small accounts collectively are important, too. Voting for witnesses is not something one should do without understanding the basics.
Steem witnesses run the backbone of the steem blockchain. Technically they are nodes that accept and confirm transactions happening inside the steem blockchain like upvoting posts, commenting, transferring of tokens and to the tiniest transaction allowable inside the chain. It is essential to support them via voting their witness campaigns or nodes or their resources. In return, you are able to do something with your stake in steem blockchain especially you operations in steemit and other steem apps. Supporting them is close to give and take situation of a two individual contributor in a system. I suggest you read more about the steem witnesses before cssting your 30 votes to 30 special witnesses.
Witnesses in principle are those community members that do extra work in keeping the blokchain alive along with other witnesses. These people are like your community managers setting up some programs that are aimed at making the steem blockchain a better place. When you vote for them, their influence over the course of what happens in the blockchain increases. The top 20 witnesses are able to decided among themselves what direction they want the blockchain to go to.
A example would be HF20 where some changes have been implemented based from the identified needs of the blockchain. Before HF20, there were some known issues, abuses, and lacking functionalities in the blockchain that people needed resolution. Witnesses will discuss among themselves how to make changes that address to this community concerns.
Ideally, you should be voting for witnesses you support that have your best interest at heart or the ecosystem. Because these roles function like community managers in the blockchain. However, not all witnesses can introduce time proven value to the community.
TL:DR - You get to have a (small) say in the blockchain plus benefit from what the witnesses are doing (whatever benefit it is). Witnesses gets paid to run Witness Node in the form of block rewards (like bitcoin mining**)
Voting for a witness is akin to voting in your country's election, municipal, state, national etc....
You can be a citizen of your country, but if you dont vote for anything, you are not exercising your rights and privilege to have a say in your country. Don't agree with your current politician? vote them out... etc etc...
It's the same on the Steem Blockchain, only much more democratic.
Asides from running Steem Blockchain Nodes that is the decentralized backbone of the Blockchain, every witness (should) bring something extra to the community, it could be added-value service, development proposal, development itself, dapps, campaigns, PR, etc etc..., Do you like what a certain witness offers or promises to offer? vote for them.
Witnesses gets paid to run their Witness Node... Kinda like "MasterNode Mining" if you squint your eyes REALLY HARD. So it benefits them to be a witness, the higher up they are the more they get paid.
**Bitcoin uses POW mining consensus, so more mining power = more earnings. Steem uses DPOS, so more votes = more earnings.
If you understand what is the role of a witness in running steem blockchain, then definitely you will get your answer to this question.
Witness runs the node and are responsible for producing the blocks and making the steem blockchain dynamic all the time and that is why you need to vote for witness, no one is forcing you whether to vote or not to vote, whether to vote x or y or z , is absolutely your choice.
The transaction and every other activity that you do in the ecosystem of steem blockchain is a result of that dynamic blockchain and which remains dynamic all the time because of the effort of witnesses and they are not doing that for free, they are also getting paid for it. But when you are voting for a witness in steem, then your are making little step to the democratic process of steem ecosystem and every such little steps will contribute to that bigger step towards democratic process of steem blockchain.
Witness is someone who (voluntarily) takes care of the blockchain ecosystem.
In this case, we provide upvote to help them (indirectly), because after all the tools, electricity, energy, time, and thoughts that they use to maintain the ecosystem STEEM blockchain must be appreciated according to our abilities.
Well, because we cannot help with tools, electricity, energy, time or thought, at least we provide support in the form of upvote, so that they can continue to become Witness and get incentives from the STEEM network.
Some witnesses reward people who voted for them by upvoting their posts regularly. This is a very solid benefit