This is a great point
@Rob93 and it has been an interesting conversation to follow. The one area where I say that 'doing good' can come into your motivations is if you are asked 'Why law over finance?' The pro bono focus in law (especially at US firms) is such that, whilst it is still a small part of the firm's overall work, it is definitely significant and can help to draw lawyers. Often 'doing good' is not even the primary motivation for lawyers I have met who love Pro Bono - they just value the exposure to something very different and interesting.
There is an even more significant point here as well, which is that law is very multi-faceted. Pro Bono aside, the work you have the potential to do at a law firm is just so broad and your training contract will give you the opportunity to really explore what is right for you!