As you probably already know, ESO will feature 50+ and 50++ content, meaning that once you reach 50 you can explore the areas and do the quests of other Factions as 50+ and 50++ which will give better gear and be harder.

What I don't get is that since you can get the same gear from endgame Dungeons and AvA, why do the 50+ and 50++ content instead of just creating a new character in the other Alliances.

If you create a new character in the other Alliances:

1) In the end of the day you get to have a level 50 character in all Alliances (should you ever want to switch)

2) You don't have to quest in some bizzaro empty instanced world.

3) You are not breaking lore and ruining the immersion by being (for example) a Nord questing in Altmer territory (which is the whole point of the PvE Faction Lock).

4) By creating alts you can try out other classes as well, instead of being the same class.

Personally, I don't get why anyone would do the 50+ and 50++ content. There so many more benefits to just creating an alt and do that content with. Even though to be honest, but the time we get to 50 we are going to be sick and tired of questing. But if you want to quest more, does it make more sense to just make a new character in the other alliances?

What do you think?

may be quests are different? just for people who have never enough questing. Also some people (like me) usually do not like play second character after first is leveled.

There will no doubt be different quests and storylines, you are entering these other faction zones as a level 50, so there will be differences in comparison to a level 20 questing in that zone.
It's made so you can see all the world with one character, like I want to do, I plan on having one main character I complete everything with and try to master, then an alt on each faction.
So yes it does make sense.

A lot of people were complaining about faction locks because they wanted to explore the entirety of Tamriel with one character specifically, so now with 50+ and 50++ you get to do that (I guess some people just don't like alts? I find it weird too). Plus, you still have to level skill lines after 50 anyway, so this just adds more content for you to do while maxing out all of your skills.

Also the same demographic of people who wanted to explore all of Tamriel on one character are probably also the ones who are new to MMOs and so, by extension, wouldn't really mind the fact that these faction zones are mostly void of other players.

Allot of players only want to play one character....

For some players identification with the realm (or even their ONLY character) they choose is very important... especially concerning pvp (realm pride).

Personally, it tend to make alts in the realm I started with my first character rather than switching the realm. But I can understand that it can become tiresome to repeat the same quests a second or third time.

I think it's a good thing, that both possibilities will be given in ESO, and that "realm hopping" (joining the winning team) concerning the pvp campaign won't be possible.