Settling retired soldiers into relatively new or sparsely populated provinces probably led to money for someone, somewhere, somehow. Having Roman settlement where it can generate produce, meat, wine, and/or tax revenue may have figured into it. If a commander / emperor had a big chunk of territory in his possession, getting the state to pay him for it so he could distribute it piecemeal to some of his former soldiers would make him popular and flush with cash.
Makes sense. A few parallels to the US, although a few different requirements, none of which for rendering actual service to society.