FillFactor is how much free space Microsoft leaves available in the index pages so that if an insert or an update happens, mainly an update, where the data grows, you don’t want those page splits to be within a row.
So, FILLFACTOR can reduce your fragmentation. Typically on systems that see a lot of updates or insert activity will need FILLFACTOR set to 80.
On reporting systems that aren’t updated as much, it’s not a big deal. On an OLTP system that gets a lot of inserts and updates it’s something you probably want to do.