Marketing, making the sales happen, etc. They wouldn't exist if they didn't do something useful.

It's like asking why affiliate marketing exists. It does and it's huge.

I suppose I was thinking about the wrong kind of product, I now realize it would be much more viable for one-time small purchases than it would be for repeated purchases or large purchases. (where the incentive to cut out the middleman would be powerful)

Find customers, take orders, provide customer service.

Everyone in the supply chain (manufacturer --> wholesaler --> retailer) specializes in their own area.

It's mentioned in the guide that wholesalers are often terrible at SEO and website design.

Hence the room in the market for niche retailers who dominate their own long tail keyword terms (SEO), offer a great sales experience (well designed site) who can identify products that are possible to resell profitably.

sell