There is no evidence that antenatal hand expression causes labour. When pregnant you will only get colostrum, once the baby is out the colostrum will change to milk over a few days.
It just means that you already have colostrum ready before you give birth. Say you have a bad time and can't nurse right away, you'll have colostrum that they can give your baby. If you have colostrum frozen then you've got a natural laxative on hand for up to a year. If you feed baby and they're still hungry then you can give colostrum and know that theyve definitely had something. It just gives you some extra preparation in case
The main difference is it can trigger labor if done when pregnant. After giving birth, it might be easier. Many do it in case they can’t nurse right away so they already have some on hand