The point here is to have advantage ... if you dont, you have to hide > therefore you have it > therefore you can sneak attack.
If you allready have it (and there is many sources to list) you dont have to hide, since conditions are fullfiled.(//Edit: Seems like i was only partialy right ... so, just read the topic before i confuse you.

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There is another threat about Sneak Attack ... i redomend to read:
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=788265&gonew=1#UNREAD