From a tourney stand point, you want loose players on your left because their action dictates what you are going to do. For example, if you have a lot of loose limpers before you, they will limp and then you can be sure you have odds to limp behind them if you wanna play say suited connectors. Or for example, if you have pocket twos and you consider limping them, if one of the loose players before you raises it up, you don't have to waste chips limping, you can dump it right there. If these players were on your left, you would limp, they would raise, and then you would have to fold, wasting valuable chips.

The value of having position on players (you have position on players on your right) is the information you receive. The players on your right act before you, and you can use the information from their bets (or non bets) to decide what you want to do.

It's nice to have tight players to your left in a tourney because it generally means that they will let go of your blinds. You don't have to worry about them raising too much because they're tight -- they won't raise or play very much anyway. So, loose players with chips to your right, tight players to your left. If the loose players to your right fold to you, you can raise it up and the tight players to your left will fold and you can steal their blinds.

Having the loose players to your left helps when you get hands too. If you notice that the players on your right have been raising a lot of hands, you know that they can't have a premium hand every single time. So with that in mind, you could potentially get paid off for monster hands when you finally get them. If someone to your right is raising every hand, you could finally call one of his raises preflop and trap him with your pocket aces. Or you could reraise him and get more money in preflop. There's lots of options, which is why it's nice to have these types of players acting before you. Position is all about extra information.

Kind of scatterbrained post by me, hopefully others will add something more general regarding this topic. Your post mentions choosing a seat, so I guess you're talking more about cash games. The same general ideas apply, but I'm sure there's more to add when it comes to ring.