Necessity, more often than not. That and being forced into a position they're not good in by ridiculous corporate rules.
My personal experience is one borne of a social anxiety disorder. Being on register for an extended period, having to help an endless line of people, that wore down my limited ability fairly quickly. They could all be friendly or neutral, and it would eat at me and within an hour I would start to panic.
I was originally a zoner/recovery person, but after being promoted to management (the company valued freight schlepping and free-reign merchandising and I excelled at both) had to take up every position in the store. Since I was lowest rung of management, I would often be used as a cashier/backup also, despite the above being made explicitly clear to my management team, store manager (there were nights he would schedule me as his cashier, often holidays, but in all fairness we also had...something of an understanding, and he stayed around the front those nights for me, or put himself in my position despite having a lot of the same problems), and higher management. It got easier in time but was never easy.
I can be good at things if I have to be, so was extremely efficient at all aspects of ringing the register, but it was still difficult after a period of time, or on off days, to interact with so many people. Retail was not for me but it was the only job I was able to get hired into at the time. I assume it's the case for other people as well. Unfortunate as it is, it's just a thing that will inevitably happen.
Schlep is such an ugly word. I think it would be beneficial to assort some insults around it. Still sad I've never had the prime opportunity to call someone a pus-leaking urinary meatus. One day. One day.