If what you mean by "necessary" is required to be done, then NO. It's honestly not that required for the blockchain to upgrade to HF20. As a matter of fact, we could literally just be content with HF19 and use it for years. This is also the reason why a supermajority of the top witnesses (17 out of 21) needs to reach a consensus first before it pushes through to know if it is really needed.
However there is no denying that there are major issues right now that prevents Steem from gaining mass adoption, one of which is the Creation of New Accounts. HF20 (codenamed Velocity) tries to provide a solution for this by their discounted account creation tokens. There is also the issue of spammers spamming the blockchain and is mostly the reason why "Resource Credits" (CR) will be introduced by the next hard fork. All are improvements that will make Steem a better and stable platform and a platform that will be ready for mass adoption.
So while it isn't really necessary to do another Hard Fork, it is however needed for long-term goals. To provide solutions to the current problems Steem is experiencing and make Steem a much more stable and great platform.
Absolutely, it is necessary since the growth of steem depends on this. With the HF20 that is planned to take place on September 25 of this year, several aspects that have flaws or need updating within the chain of steem blocks will be improved, some changes that will occur during the HF20 are:
to mention a few, (you can see more about the HF20 from the following link: https://steemit.com/steem/@steemitblog/steem-velocity-hardfork-hardfork-20
In addition, the HF20 is also necessary to receive the SMTs that will add value to the chain of steem blocks through the different DAPPS that are planned to be created or that are already created and are waiting for the development of SMT to be completed, which will happen in the first quarter of next year.