Certainly, there are many big dudes out there who dont log their shit. Certainly, many people can remember what they lifted the week prior. My opinion isnt a steadfast truth. Just an observation Ive made.
And you dont need to always do the same scheme (3x8, is the example given above). Just because one week you did 5x5 and the next you did 3x8, then you did 6x4, etc... that's fine. You can still get a great idea of your progress, especially if you keep track of it for multiple weeks.
Example: "Today is chest day. hmm... Im sick of the 3x8, im gonna move heavier and go 5x5. I remember doing that last month. Let's see what weight was moved..." Now you know about where you should be +/- a few pounds, instead of guestimating on the first set or two and then zeroing in on the optimum weight on your last set, because you havent done 5x5 for 2 months and you have no idea where you should be (you have been doing DBs or was doing 4x10 or something):
1 - 225x9 -- way off
2 - 245x7 -- getting closer
3 - 255x6 -- almost
4 - 260x5 -- there we go
5 - 260x5
While the first couple of sets werent "wasted", they could have been put to more efficient use in a 5x5 scheme. May as well have used the 225 as a warm up set. So maybe you shoulda done 1 more set of 265.
To me, it's just so much easier to look back in your log. You'll instantaneously know what weight should be used, if the previous weight was too heavy, too light, etc, and you'll be able to track your progress. "Sweet, two weeks ago I did 265x3, and now im using it in my 5x5."
Also, another thing that's useful is writing notes about intensity. It can help keep you accountable, in a way. If I was a pussy that set and didnt put out as much as I could/should have, then I write a big P and circle it next to that set. And apparently Im self-conscious of myself, because that (P) helps motivate me. I try to avoid having that symbol, naturally. At the end, I usually write a quick note: "Tough workout, bit drained on incline flies, felt great on incline DB though." Something like that.
One key thing is to not let the log dictate what you should do. I know a few people who do that.
In the end, I just think it paints a very clear picture of your progress and helps clear up your future lifts. Not to mention, it's fun to look back. It's my bible.