I been reading a lotta strategy poker books. maybe I'm just slow but I don't get expected value. how do you calculate something your not sure of , like your opponents hand? all I use is my outs, and pot odds. somebody explain to me why you'd wanna use expected value? I just think some GTO book writers came up with another way for you NOT to fold
To calculate EV definitively you need all the data, including your opponents' holdings.
Therefore it is a very good analysis tool when reviewing play retrospectively.
I.E. to look back and see whether any given decision was positive, or negative, expected value.
As you've noted, during play we don't have all the information required and, even if we did, working out the EV formula is above most people's mental arithmetic capability in the short time we have to act.
That being said, with practice and experience you develop a better sense of what your EV is going to be like in certain spots as well as what kind of plays will generally result in positive or negative EV.
While it might not be something you calculate at the felts, understanding the implications of EV and learning to invest chips in +EV spots and avoid -EV plays is absolutely crucial to profiting consistently at poker.