Interesting - I'd reckon my boarding pass is possibly re-checked about one in 3 times - and always after I've made a mental note of the seat number and put it away in order to manage mybags/jacket etc.
I always thought (in common with a lot of other SLF I guess) that you wanted to know so that you could (helpfully) tell me my seat was the window one 5 rows down. We (helpfully) tell the CC our seat number, to indicate that we know already!