Basically that is like asking what benefit do you get from buying Facebook stock if you don't use Facebook...
If you buy STEEM but you don't intend to use Steemit.com your only benefit would be the increase in value of STEEM over time. The nice thing about STEEM is it is much better than owning stock of a company because it can also be used as a currency, you can buy anything with STEEM anyone else is willing to sell you or provide a service for you.
Not only that, authors of content are not the only ones who get rewarded on Steemit.com, the people who upvote posts get rewards too. Currently the author gets 75% of the value of the post while all the people who upvoted the post split the remaining 25% of the value. So the 25% is split among everyone who upvoted the post, but the amount you get is determined by how valuable your upvote is.
So imagine if on reddit every time you upvoted a post you got paid for it, it is not much but essentially you are being paid for participating and using the site.
So if you dont plan on posting anything on steemit.com I would highly recommend creating an account because you can still earn a tiny profit just from upvoting the content of others. This grows the user count of the site, which in turn increases the value of the site, which in turn increases the value of STEEM.
source: https://www.reddit.com/r/steem/comments/7l0r6j/what_benefit_does_steem_have_to_outsiders/