They bring new people to steem everyday. When you tell people to make posts on steem, they see it as something meant for professionals, but a dapp like musing.io that allows people so ask questions and also answer questions, they feel they can do. After a time, they realise that it's not that difficult and they join fully.
There are a few ways Dapps help Steem. Firstly the Dapps that are built on top of the Steem Block chain add value.
What I mean by that as they tokenize their sites or Dapps they are held against Steem. If we look at Musing and they launch a token this token would hold value against Steem. The more users that hold and use the token the higher the value would become.
Imagine in 5 years time and there are over 1000 Dapps all holding tokens valued against Steem. Steem only has 330 Million coins and maillions could be held by Dapps. This would create a shortage or scarcity which would make Steem more valuable.
The Dapps have an onboarding process allowing them to invite new users and opening a new Steem account. Steemhunt and Steem Monsters have been doing this for some time and it brings flesh blood onto the block chain through another entry point. At the time those users may not be aware of Steem but it wont take them long to understand how everything is linked.
Dapps are the future for Steem to grow in value and for attracting more users. This will take some time, but the block chain is growing daily as new development happens.