Alright so this needs a more in-depth explanation.
First thing you should understand is the power difference between Pokemon. There are simply Pokemon that are better than others; for example, Infernape is more useful than Emboar because it actually has a good Speed stat, it can run viable mixed sets and use scouting moves (U-Turn), all of which Emboar lacks. The existence of a better Pokemon hampers Emboars usage and its slowness means various bulky Water Pokemon (Swampert, Vaporeon, etc.) can revenge kill it easily, thus it ends up in a lower tier. Infernape has flaws too (e.g. things like Azumarill and Mega Venusaur stopping it), which is what kept it out of OU this Generation.
>but baton passers and certain item can make weaker Pokemon stronger too!
commented 40 minutes ago by Fuzzy Kawaii Raichu
There are problems with this statement. You're saying that, say, a Choice Band would make Garbodor as good a Zekrom? Bring that Garbodor into an Ubers match, I guarantee you it will be obliterated. Its defences cannot take attacks from the Ubers tier's powerful attackers well at all. And what if stronger Pokemon can use those same items? If I put a Choice Band on the Zekrom that makes your point even more invalid.
Tiers basically grade Pokemon based on viability (NOT popularity) to keep things balanced and prevent weaker Pokemon from being demolished by stronger ones. Anything RU and above is banned from NU, because thousands of battles on Showdown have proven that the Pokemon in those higher tiers are too strong for the NU Pokemon and bring an imbalance to the game (or achieve a certain amount of usage in a higher tier proving their worth). If a Pokemon in a tier is decided as too strong, they will ban it from that tier and move it to one above. It's a simple concept and it works. Also, don't get the idea that it's all opinion based; you can see the usage stats that a Smogon member named Antar collects each month, based on each and every battle fought on Showdown. Just search "OU usage stats" and you will see examples of this.
>Do I really do I have to select one of those certain types of Pokemon? What happened to free battles?
No you don't, you can use any Pokemon in a lower tier than the one you're playing in, which means if you play in Ubers you can use anything you like. If you do that, prepare to get wrecked, though. I also believe there are separate metagames for these "free" battles on Showdown, otherwise just stick with ingame because competitive battles don't agree with your current opinion.
If you want to play a game where all the overpowered legendaries and Megas are banned, play OU. All the other Pokemon and less powerful legendaries and Megas are allowed in OU.
tl;dr: they keep the game balanced.