Were they trying to help Germany make their war reparations payments? Was this a roundabout way of helping Wall Street recuperate their war loans to the Allies? What was the reasoning behind it?
The US wasn't really that interested in collecting war reparations from Germany, especially after their currency spiraled out of control and became worthless. However, the US was definitely interested in getting back the massive debt owed by the UK and France. The Dawes Plan provided a nice way of doing that, since getting back the debt owed by the UK and France was contingent on Germany repaying its war debts to them.
The Dawes Plan was more than just a loan, as it also fundamentally restructured Germany's payment plan for war reparations so that it would actually be possible for Germany to pay their reparations after their financial collapse in '22. The Dawes Plan would serve to help stabilize the German economy, allowing them to pay war reparations, which would in turn allow the UK and France to pay off war debts to the United States, and the US banks would eventually get the money they invested into the Dawes Plan back from Germany.
All in all, the Dawes Plan was pretty successful at mitigating the war reparations crisis. It did have some undesirable side effects on the German economy a few years down the road related to their dependence on foreign markets, but that didn't matter too much to the US.