Most people want to feel like they are good, kind. Preventing harm to something much smaller/​weaker than themselves reinforces this. Even better if it requires very little effort.
Social signal. I personally immediately trust people more if they take their spiders outside rather than kill them. I think they’re more likely to have good intentions in whatever else they do. I think many people feel the same way and are vaguely aware that carrying themselves like this sends a useful signal to others.
I think it serves 2 purposes:
Most people want to feel like they are good, kind. Preventing harm to something much smaller/​weaker than themselves reinforces this. Even better if it requires very little effort.
Social signal. I personally immediately trust people more if they take their spiders outside rather than kill them. I think they’re more likely to have good intentions in whatever else they do. I think many people feel the same way and are vaguely aware that carrying themselves like this sends a useful signal to others.