One thing to remember about sponsored magic is that it advances the sponsor's objectives - not necessarily the PC's.
The longest running example from the books is Hellfire. The powers of hell were happy to give Harry all the Hellfire he wanted, to do anything he wanted, just for the compels. Then the sponsor used that debt and offered compels to Harry's player - things "lose your tempter" or "be suspicious of people" or "piss off your allies". Re-read the books with the game in mind and you can practically see those compels being used. Slowly but surely those compels were changing how Harry acted.
With Soulfire... compare the books where he has Soulfire with those where he has Hellfire and you'll see that Harry is doing more acts of random kindness. Touching people's lives in good ways. Not that he's not generally a nice guy but after he get Soulfire he makes a bit more time to help others. Until we reach the latest book (GS Spoilers):
where some gangbanger types shoot up a house that his friends are in - critically injuring one of them while killing a bystander - and his reaction is "How can I help get those boys out of that bad situation?"
So when looking at "what can X type of sponsored magic do" you have to ask "what does the sponsor want done" and "how does the sponsor want you to pay for it". And it can be critically important the player and GM are on the same page for that last bot.
Richard