Where do the prentices sleep? Why does Dickens add this detail??

I can't find the exact part you are referring to. I assume the apprentices sleep at Fezziwigg's business so they can save money.

The apprentices sleep in "beds; which were under a counter in the back-shop." Dickens adds this detail because the company that Scrooge worked for was similar to the one him and Marley have. Therefore if they want to be lending out money, they need to save money by not paying for the apprentices' accommodation. Hence why they are sleeping in the actual store they work in.