On an open fast boat keeping a "ships log" as you go along is not always practical, but as you've heard above a lot of people are keeping some sort of log for their own / evidential purposes that they complete after the event.
Traditionally a ships log has been a useful tool on vessels undertaking reasonably long passages to help with tracking where you are - heading, speed and time, allow you to "dead reckon" a position based on a known starting point. To some extent the modern world of GPS navigation may have reduced the need for this but electronics are not infallible, particularly on a bouncy open boat.
A log which records fuel usage is also helpful in both determining fuel consumption and, as you go along, in estimating the endurance you have left. On a short trip you might get away with these things in your head, but over 5 or 6 hours you can start to forget details, and especially gradual changes.