Why would you need to ask for a libro De reclamaciones in shop or restaurant?

Libros de Reclamaciones are Claim Forms. You would file them if the shop's employees or the restaurant's employees mistreated you in some way so that you could file a legal claim against them. Most shop-owners and restauranteurs would prefer to simply provide redress for the problem rather than go through the rigamarole of the Reclamacion process.

For example, imagine that you went to a dry cleaner's and he refuses to give you your shirts after you pay him. You can demand an Hoja de Reclamaciones (a singular claim form) in order to file a claim against the dry cleaner for failing to give you your clothes.