It might be cheaper in some settings.

For certain food styles, I buy bulk spices sometimes because I don't like to pay for an entire jar I won't use, knowing that most of it will go stale by the time I'm through the jar. Being able to buy tiny quantities is sometimes way cheaper.

I'm also mismatched in my conditioner and shampoo remaining where I can buy the matching set and let the difference persist, or I can try to buy a single catch-up bottle of whatever I have excess of, to hope that they even out by the time I get to the bottom of a bottle.

Basically, I can imagine where it might be preferable (for both cost and convenience) to buy an arbitrary amount of something rather than buy a fixed factory container of that thing. I know I already do it for certain things.