I'm no pro, so take this for what it's worth. First off, in all smash games, and all fighting games, all characters are not created equal. No matter how hard you try to balance them, some will end up being better, whether just by their nature, like speed or strength, or something overlooked, like chaingrabs. Anyway, it's a human tendency to rank things. Hence, the idea of tiers. The point isn't to tell people who to main. It's simply to determine which characters are best in the singles tournament setting, using the input of the best players (the SBR).
The types of tiers change for pretty much the same reason that the tier list itself changes. The metagame constantly evolves. New tactics are found, characters rise and fade, and things need re-evaluation. Thus, the tier list is updated when the SBR feels it's time. The different tiers reflect points where the SBR see clear, or at least clearer, gaps between overall character goodness. As the metagame evolves, and the list changes, things must be re-evaluated. This one has 4 sections because the SBR feels that those are the best places to break it up. There aren't rules like "Each tier must have 7 characters" or "There must be 5 tiers" or the like. The system is intentionally fluid so metagame changes can be accounted for.
Again, this is all just the observations of a non-pro observer. Take it as that, and if something's horribly wrong with my logic, please don't kill me.
Oh, and welcome!