To be fair, at this point a law plainly stating that everyone, domestically and abroad, can legally be subjected to whatever surveillance is deemed desirable by state agencies for no reason whatsoever wouldn't really change anything in practice, would it?
It will make it more difficult to fight in court. Currently, surveillance could be judged illegal, and the FISA justification thrown out. With an explicit bill, it becomes definitively legal, and fighting it requires the US Supreme Court to judge the law itself unconstitutional. It's also exceedingly difficult to get spying cases to the US Supreme Court, because getting there requires proving harm, and it's rather difficult to prove harm when you can't prove the classified spying is even taking place.