Comedy, yes. He's the one left to do things the others are probably too old for. (.. I send my sister over to get/do something sometimes because she's "younger and it wouldn't look as odd than if I had done it".) He did get his character built a little in the 6th Pokemon movie with Jirachi. And because he knows a thing or two about Pokemon, he can easily identify a Pokemon the group has probably never seen before. (Usohachii, 8th movie.) He's usually the one playing with the Pokemon, and is probably the one to most likely be the first to know where a Pokemon went missing. Or where something disappeared off to. Or what somebody's up to, in a no good manner. Because Masato is still a kid, and little, people probably wouldn't really notice he went missing, so he can be like a little spy. XD
Otherwise, he works as another character the anime can focus the fillers on to build character when we've already seen too much of Satoshi. (Not that I EVER get tired of him. <3) I do enjoy the Masato/Satoshi moments, such as in the third episode of AG, as well as the movie. Yes.
EDIT: I re-read the thread. I'd like to add I based what I said on what I remembered from the 8th movie, as well as probably some of the older AG episodes, and maybe from the newer. I don't remember. =B Oh yeah, don't you guys notice.. He feeds Gonbe his/her(?) Pokeblock so that it doesn't go around pilfering others' food? XDD
More in the thread
http://pokecommunity.com/showthread.php?t=58324, thanks for scanlating! XD