Wizardkind with older family names are more powerful than younger families, orphaned wizards as their power is pooled together as time goes by, a wizard's power is added to the family line when they die etc. So a wizard like Graves or Picquery has additional magic to fall back on besides their inborn magic, which makes them really powerful. If you marry into a more powerful family you get to tap into the family power

The reason why Grindelwald is so obsessed with finding the obscurial? He needs the additional power as he has been cut off from the family power after he went Dark Lord and got disowned.He's been experimenting with forced bonds but the witches/wizards he bonds with tend to die after a while. Forced bonds don't seem to turn out well for the forced wizard.

So cue events of movie, Graves is currently forcibly bonded with Grindelwald, unwillingly supplying him with power while he waltzes around with Graves face. Will he manage to break free before he’s sucked dry?

+ Graves chooses to suicide in a sense by magically breaking off all familial ties. Grindelwald only has Percival inborn power left to feed on, though it’s still a lot

++ Graves is rescued but the bond is still there, MACUSA tries to break it

+++ Grindelwald manages to bond with Credence but it somehow backfires and the obscurial bonds with Graves instead and breaks the bond between Graves & Grindelwald. Credence and Graves are now accidentally married