He's the star of your deck, his type and attack costs determine what pokemon you can run, with him you can KO a quicker victory than with just normal pokemon. He is also always available given the proper resources. Each time he get's KO'd it takes a little more effort to bring him back.
In magic the generals colors determine what you can put into your deck. If you do damage with him enough to one player, it will make that person lose quicker than by dying than with normal damage from other creatures. You can always access him, by paying the cost in order to play him, each time you do this it costs a little more than before.
The legend rule is a rule in any kind of format for mtg, it was simply set in place so that it makes sense because you would obviously see a bunch of goblins all the time in mtg, but why would you see the same legendary named individual more than once? Pokemon sort of ignores this idea in almost all of its applications, it also seems odd to implement it into pokemon. The legend rule isn't something that's particularly associated with EDH.