The general idea is that they would make OU less competitive as they would be too powerful, so most if not all teams would be required to have them. I don't follow closely enough to know why specifically each Pokémon is overpowered though, if there are any in particular you're curious about I'm sure you could look on Smogon's website or look for Youtube videos explaining.
Just for comparison, last month OU's top 3 Pokémon had 30%, 27%, and 23% usage, while Ubers (the tier where any Pokémon can be used regardless of bans) had 70%, 63%, and 35%, showing how much less balanced it is.
Just for comparison, last month OU's top 3 Pokémon had 30%, 27%, and 23% usage, while Ubers (the tier where any Pokémon can be used regardless of bans) had 70%, 63%, and 35%, showing how much less balanced it is.