In my family it's always been normal for over 100 years, although not expected or asked for.
I think a good way to explain it is, imagine the parent has 10M in bank. Meanwhile the child has nothing, and is in their first job at 30k a year.
It feels weird for the parent to see their child have a vastly different lfestyle to what they have, e.g. parent could go on unlimited holidays a year, the child perhaps goes on no holidays until they improve their position. Parents wants to go on holiday with child, so they pay. Or the child lives in terrible, unsafe housing, while the parent lives in a mini mansion, so again the parent might want to step in.
Equally, there was a point in my 20s when I was significantly better off than my parents, so I would take them on holidays etc. i know lots of kids that buy their parents homes, take them on holiday etc.
It's love, really.
Personally if I saw anyone I love struggle and be in a much better positon than them, whether it is a partner, friend, sibling, parent, I would help them. Or if I had much more time than them and they'd be rushed off their feet, I'd offer to help in some way. For example, one of my friends is having a tough time with childcare, and I have lots of spare time, so I do occassional childcare for them just because. If it was my child instead of my friend, I'd of course do the same
Most of mum's friends help their adult children in some way and take immense pride in it.