They don’t! As web applications are more and more packed with information, the need to hide controls have emerged. The option would be to have even longer web pages, showing a lot of redundant controls for every post as in "unfollow post, unfollow updates from user X, unlike page, still like page but don't show updates, and on and on and on.
This has made designers hide controls and additional information which you can only access on hover. Sometimes these controls are in very long sequences on hover leading to hover leading to hover which if you eventually miss a spot, lose all the sequence in once and the user have to start over.
This leads to great cognitive load on the user making them frustrated and possibly leaving the site because they can’t handle it. And how do you hover from a tablet or smart phone web browser? This is one of the unsolved mysteries that web designers and UX professionals should deal with and make conscious choices of every step.
This is why we often access content from various sources: Desktop Computer, Tablet browser, Tablet App, Smartphone browser and Smartphone App. All of these is a part of cross channeling where users have to accept the fact that they can’t do everything on every device. I’m struggling with if this I the right choice almost every week.
The idea is that a complete experience can be realised by jumping between channels at will.
For instance, a module on Task Analysis that is designed in such way that learners can choose a different media for each sub-module, and regardless the media choice their experience will be complete.
The authors do mention that it may not be possible to align all the steps between the various channels.
Content from the accepted answer to the question What is Cross-Channel?