Coder preference , mostly. It is nearly impossible to judge code w/o knowing it's intended purpose, but I am assuming this was just part of standardizing reset CSS.
SOME (not all) CSS properties , such as list-style for example, are inherited by descent elements. so setting the list-style for the UL is tantamount to setting to its LI children.
Padding and margin are NOT inherited and am not sure if there was ever a time when the indent on a list was created from the default margin on the LI element, but that might account for what you are seeing.