I've watched a lot of speedrunning/im decently new and I've seen a lot of people destroying the portal with a water/lava bucket upon reaching the stronghold. My guess is it's to not mess up with the preemptive, but I'm not sure!

you guess it

If you have a block highlighted that will increase the unspecified (green part) of pie for preemptive, and you're intended to use the ratio between unspecified and blockentities (orange part). So people will break the portal so it's easier to avoid highlighting blocks. A lot of people just do this because they see other people do it, but if you're going to be inconsiderate of block highlighting or the ratio between unspecified and blockentities then there's no point in doing it.

pigeon