I know people do it very occasionally, but when was the last time you purchased a gig or theatre ticket on a whim, and not at least a day or two in advance when you could, having done so, reserve a seat on the relevant train? Mostly there will only be one such train anyway - the railway isn't known for a frequent service after 2300.
The point with relation to reservations and this sort of travel is that events like this have slightly vague finishing times, which are not purely running-time isseus eg they connect to things like how long it takes to get out of the venue, get bags from clokerooms etc. Yes, you can always book on the absolute latest train to allow for any delays but then that is clearly less convenient than knowing that you might be able to get an earlier service in the event of a quick getaway. And that will push some people towards other options: bus, taxi, driving.