What the others have all said, but mainly what ItsMeToo said - it's fun.
I quite get that some people only play the game for the PvP and hate "grinding" the PvE, but I have no interest in PvP and have never considered leveling through PvE to be "grinding".
By playing a number of characters - I currently have 12 over two accounts - I can vary what I'm doing and never get burnt out or run out of content.
This is a good discussion regarding the thoughts behind which alts players have created. I have (3) alts - 1 in each alliance. As I have started playing a bit more seriously, I wondered if I should of made (3) alts in one alliance?
Also for different factions, especially if you have new or old buddies there you want to play with. Can also be pretty interesting to check out different versions of Cadwells silver/gold.
8 character slots is not enough to explore all classes, favourite races and check out 3 factions. If they ever start selling character slots, they will for sure make money.
This question has been bugging me for a couple of days. Let me explain by doing a brief (very brief) explanation of multiple characters in previous TES titles:
Morrowind: played with my Dad, maybe rolled four characters, played to the end, followed main quest. Probably less than 100 hours of play time.
Oblivion: Multiple, multiple characters. Roleplay, immersion, experimentation, etc. Over 500+ hours of play time. Used characters to explore my imagination and the open world.
Skyrim: Again, multiple characters. Open world, with different regions. I could go and do what I wanted. Played main quest; ignored main quest. Rolled a Dawnguard character. Rolled a Companions character. You get the idea.
I was excited, when I bought ESO, to do that same thing - except I feel like I can't. There is no open world - at all! So every character starts exactly the same way and lands in one of three zones which will be achingly boring to play through after I get to V levels in three characters. I'll be like ... "This quest ... again .... sigh."
I like playing alts; I like having characters with different backgrounds. But then the only thing to do is crawl the same, boring pain-train quest line and do the same things again and again and again and again ... like Bill Murray in Groundhog Day.
I'm just having a hard time getting excited about alts in this game.
War does not determine who is right - only who is left.
The heroes during times of war, are nothing but mass murderers during times of peace.
Riza Hawkeye
Learn to play, or resign to become one of pieces that is meant to be sacrificed.