To kick ideas off of, mainly.
If you let co-founder in you are not only sharing % of the company but also vision. Getting co-founder is like getting married, if you struggle to find someone you know and trust maybe it's better to stick with advisor for now.
But sometimes you need someone emotionally invested in it. Cofounders are much better. They don't necessarily have to have 50% share. I'd be happy to take on someone whose sole purpose is to be the customer surrogate and give them a 20% share.
My cofounder helped to keep a leash on me doing really dumb things. All good entrepreneurs are relentless, and a cofounder helps to keep it in perspective. Also a cofounder adds their network, and my minority share co-founder landed the acquisition.