What happens if I get Mama Murphy off chems?

Mama Murphy's visions give you some cryptic allusions to events which will happen during the main quest of the game. But there are 5 visions which do indeed give you useful hints, unlock new dialog options and one which gives you a passive buff during a boss fight. The following information is taken from the article The Sight from the Fallout Wiki on Wikia:

Vision 2:

Unlocks a dialog option to avoid the fight with Skinny Malone and his cohort while you rescue Nick Valentine from Park Street Station / Vault 114

Vision 3 and 4:

3 tells you where to find a key to Kellog's house in Diamond City and 4 tells you what to do in there

Vision 5:

25% damage reduction during the fight with Kellog

Vision 7:

Unlocks a dialog option to avoid a fight with/for Z2-47 by using his factory reset code

Note that you can only receive up to 5 visions in total per playthrough, because:

After the 5th vision, Mama Murphy will overdose and die. The other characters in Sanctuary will not like that.

Apart from providing clues about the main plot and making some quests slightly easier, Mama Murphy doesn't affect game play. Giving her drugs might however affect your relation with companions.

So getting her off chems provides no other benefit than companion relation boosts, as well as her remaining a settler and working in the settlement (by default she works to improve mood of other settlers). You will get a few unique voice comments from her whenever you encounter her.

The opposite, getting her killed by delivering too many chems to her, will make Marcy (one of the other NPCs that followed Preston) spew some angry and upset comments at you instead.

I wouldn't worry much about her, as the sights provided are quite cryptic and you can solve the main quest just fine without them.

Full story + spoilers at the wiki: http://fallout.wikia.com/wiki/Mama_Murphy