It definitely benefited Justin Sun to vote for his Top20 Steem witnesses.
Top 20 witnesses can do good or real bad to the platform. They have the power of accepting changes to the code of the blockchain which dictate which rules the blockchain will be following. By voting for witnesses you trust (up to 30, which can be updated at any time), you participate into electing who can have the power to defend your interests or attack them.

However, there is no direct benefits, you won't earn more, you won't be VIP in their curation, you won't escape their downvotes etc...