Some Dive Location operators may require a log as proof of experience. I know logs can be faked but that’s still on the diver. I use a log I built and it allows me to log more quickly and with more detail. It also does all the SAC calculations for me.
Dive computers are now getting good at holding lots of details so manual dive logs may become a thing of the past.
As Justin stated having information about a dive location maybe beneficial to you when coming back to the location or to a similar one.