Takes me about 20 minutes once a month to compare my bank's monthly PDF with my own records, and update anything I missed. One reason not mentioned so far is to identify payments I don't recognize, just in case someone got access to my account or a monthly charge suddenly tripled.
So far nothing big (some things on Amazon change price by a few dollars up or down after putting in the order), but I always wince a little when I see someone not bothering at all.