I find them to be very useful especially when I am checking out banquet venues and talking to the sales managers. You wouldn't want to leave the banquet venue then realise that you didn't ask some important questions. It also help if you ask the same set of questions (or at least similar ones) so that you can make a fair comparison when deciding on the ultimate banquet venue.
I have found this very useful youtube tutorial by starsinpalm on how to make and customise your own planner. If you are someone who loves to DIY, or you want to make a customised planner as a surprise for your SO, check out this video!