Some use them to have power. Others use them because they geniunely like them. And I'm guessing that some use them simply because they're sick of people who go "peple who use legends suck they're bad trainers it's how u train the Pokemon that counts and if yur frends with it!"
I fall in the second catagory. I like certain legendaries, and there's nothing wrong with that. If I want to have all the legends I like on one team, there's nothing wrong with that either. It's not like this hypothetical team of Groudon, Entei, Ho-oh, Zapdos, Rayquaza, and Latias would be something YOU'D have to deal with, so what's wrong with it? Aside from the fact that four of them are weak to ice and four are weak to Rock.
It's not weak unless you're just using legends for the heck of it and have no clue what you're doing. Legends aren't bad just because they have high stats, and I honestly don't get where the general negativity towards using them comes from. Just because YOU'D rather not use them doesn't mean that someone who DOES use them is horrible. I'd never use a mono-type team, but if you want to I'm not going to insult you for it. If you have a problem with legends, then don't use them and shut up about them. You may want the challenge of not using them, but I want the freedom to use whatever I want. The games are easy as pie if you bother to level up enough anyway, so it's not like using Legendaries in-game is going to shatter the difficulty level completely. Competitively, they're in tiers appropriate for each one so the "they're so much stronger than most other Pokemon it's not fair!" argument doesn't hold up unless you decide to ignore tiers.
Or should I start on a rant about why I hate one of YOUR favorite Pokemon and deem you weak for using it? I very well could, you know, especially since EVERY POKEMON has a weak point to it, no matter how strong it may be. There are no invincible Pokemon, especially when considered individually. Even if you only use strong Pokemon, you still need skills to do well with them, just as you would with weak Pokemon. There's no getting around that no matter what you use.