It's on by default and seems to do very little.
Websites have more and more ads now and it definitely spoils browsing. I tried a proxy ad blocker on my old i5 but it slowed down browsing a little, so I haven't bothered with it on my 6+.
I can't say I've really seen any pop-ups with Safari. Pop-overs yes, but that's a different thing, and short of some sort of ad-blocking functionality (through a proxy or something lien that) I can't say I've seen many third party browsers offer something to deal with that either.