There's a big difference between winging it which takes more confidence than skill, and doing the job well.
In the blues band its great for confidence building, because there's a common musical language and conventions, even if you aren’t playing a twelve-bar. For example, its rare to have a true bridge. This means when on unfamiliar territory you can still anticipate enough to sound reasonably tight, and when you do know the material you can go off piste with confidence.
In the covers band even something as simple as Chasing Cars - sure you could probably play it without having heard it before, but to do it well there's a fair bit of dynamics and stop start to learn. A better example would be Purple Rain where you have to build your playing over the song and can be fairly improvisational, but you need to know when and what you're building up to.
Of course there are songs you can learn on a single pass, but on clser inspection most turn out to have subtlety you need to tease out to do them justice.
If it was really that easy, why is such a large percentage of onlne tab wrong - I recently got a site to correct its "offical tab" for Green Onions FFS!