Pet owners are among the most regular customers in any trade. Food runs out monthly, litter every two weeks, and vaccinations have dates. The problem is that they buy from the nearest shop when supplies run out, not from the shop they prefer.
A monthly food order solves that simply: the customer sets the food and quantity once, and the order renews itself. They stop having to think about it, and you secure a customer every month instead of competing for them each time.
If you offer services such as grooming or vet visits, a file per pet with vaccinations and weight lets you remind the owner at the right time. A message that the pet's vaccination is due next week brings a visit, and builds the trust that brings all of the pet's needs to you.