It a very common story telling techniques, someone need to be clueless and so that the smart character have to explain stuffs to them and by extent the audiences. That especially Watson original purpose, an audiences surrogates for Sherlock to explain things to, or else it would look like Sherlock just saying these thing to himself when he have no reason to do so.
I know it's common, that's why I'm bored of it. And it's not necessary. Conan doesn't keep a stupid sidekick around to monologue to, he just explains his reasoning to the police (or anyone else who happens to be around really). So does Kindaichi.
And even if you want to have an audience stand-in, there's no need to make them an obnoxious moron. The girl from Phantom Seer serves the same purpose, but she's not constantly screaming about how unbelievable everything is (even though she has a lot more reason to, really). The Watson has to know less than the detective guy, but s/he doesn't need to be
stupid, and there's no reason for the detective to be a recluse either. Those are just tired cliches that too many manga keep repeating for some reason.