beckham wanted that - theres just no other reason he would come
juan pablo angel also did that, he kinda resurrected his career too
I dont know if you know this but in MLS players are owned by the league, not by the clubs, so the league pays most of the wages
in MLS theres a salary cap, that way no team can run into financial trouble and no team will pay too much
but there is a DESIGNATED PLAYER, whos wage until the cap will be paid for by the league, and the rest will be paid for by the team
David Beckham, Juan Pablo Angel etc. are all like that
Abel Xavier, Danny Dichio, Ronnie O'Brien (maybe the Irish people on here know him) arent such players, so its obvious they came here as a normal transfer - to get more playing time, to become a better player
not just to finish off their careers when they cant even run (not angel, i mean only beckham), let alone play well - and get paid millions | 
