I have never done this.

The amount of time that I spend making digital purchases is low. Having an agent do things wouldn't save me much time even if I could completely trust it. I would still need to interact with this agent for item selection and I'd need to babysit it for correctness. For things that I buy regularly basically every service has an option for regular purchases.

I genuinely cannot conceive of a circumstance where having an agent make purchases for me would improve my life.

That is a fair critique. If you don't spend much time buying things and/or enjoy shopping, then there isn't any need/pain point there.

I think there are a group of consumers out there that do have a pain point in that there are things they want/need to buy and hate the shopping part of it. I know I fall into that category. The issue with the regular/recurring purchases is it needs to be exactly the same thing/amount on a regular time interval. For me at least, I either end up stacking up more than what I need or run out before it gets there.

I think the business demand is a completely other beast in an of itself. Ranging from solopreneurs/small companies who have agents running a all or part of their business to big business wanting to replace the people in the accounts payable department.