Why anyone can be represented on a disability or ssi claim

It occurred to me at one point (and it has reoccurred to me many times since then) that, in the U.S., one's ability to seek and derive justice, and one's treatment in the justice system itself, may be mediated by one's finance's, i.e. ability to pay upfront. Getting assistance on a traffic case is one example of this phenomenon. If you have the cash (and depending on the infraction and the state