What do you mean what was the point? Isn't it somewhat obvious that the storyline about the monolith and what it represents hasn't ended, we're probably just in the beginning of it? Her being where ever she was will probably not be just shrugged off. It also serves as to damaging her psyche while also toughening her up, so she's a different person now than when she disappeared, which has the potential to make for some pretty good character development & interactions while also giving her some fresh set of skills that she didn't have before, giving her slightly expanded possibilities as far as potential storylines go. She's not necessarily the Simmons who'd need Bobbi to save her anymore.
We'll obviously hear what she's been doing all this time and given that there was a few month time-jump from S2's end to S3's beginning, there could be quite a lot of things to go through there, things that could end up either becoming new, major story threads of their own as the season advances or, perhaps more likely, stuff that will end up tying to the emergence of the Inhumans. And I wouldn't be surprised if Simmons being there and escaping means that something else might come to Earth as well, either in pursuit of her specifically or just to Earth more generally because someone from Earth was there and escaped. Simmons was clearly running from something she knew was dangerous in Epi 1 and I wouldn't be so certain we have necessarily "seen" the last of whatever it was. Somehow, for some reason, it might make its way to Earth. The monolith might be destroyed, but what it represents could be one of the big story threads of this season, something on the level of the Inhumans reveal & all the Inhumans shit going down in S2.
If anything, Jemma being back hopefully means that we'll be introduced to those bigger storylines that are the driving force of this season sooner and those threads will have more time to be developed. Instead of dragging the mystery for half a season.