Co-GMs can be useful for a lot of things, though it depends on the scope and needs of your roleplay.

If it's a small group thing then it's not really that useful unless you're frequently absent and need someone to hold down the fort when you can't be around.

If you're running with something larger, say getting up into the double digits for player count, it can be nice to have someone else to answer questions and deal with issues so you don't have to do it all yourself.

If your roleplay covers a huge area with tons of characters, co-GMs can be great to have to play some important NPCs and such. It adds a different feel to different NPCs, which is nice for the player side, and that means the GM workload is shared, which is nice for the GM side.

If you're bad about reminding people to post, having a co-GM just for whip cracking purposes can be nice. I've played that role myself a couple times, and having someone whose job is to tell people they should post whenever they're holding things up tends to be pretty helpful for pacing.

It all depends on the RP and your availability, really. Some people can manage massive games with tons of people without any issues, others need the helping hand of a co-GM. Some people are great co-GMs, some people suck at it. There's no single answer for any of it that'll apply to all games or people.