Better guest experience? I know for a fact this has all been on this forum before. Better experience > more likely to visit again > more likely to recommend to others = potential for more visitors. Staying open an extra hour you have more chance that guests might spend that final part of the day spending money on merch, spending money on food, spending money on drinks; etc. More spending = more profit = the point of staying open longer. Same thing applies to a 6, 7, 8, 9pm close. If 10-4 is your only visit of the year, which for some people it will be, they might take food in and eat it in queues because they've got to rush from ride to ride - not because of queue lengths, because of the sheer size of Alton Towers.
It's nothing to do with reputation, but you want to give your guests the best possible experience. Also, they aren't necessarily dead in a COVID world, the 4pm closes earlier in the year that everyone "thought would be dead", were busy and unwarranted. Some remained at 4pm days before I visited, when they shouldn't have, and were only extended post my visit. Plenty of "point" to banish 4pm closes altogether, which as I've admitted, they've mostly done a good job of.
Just because they've been 90/95% great all year, it doesn't mean that last 5% can't be critiqued where its warranted.