Does your company have customer and order information in more than one location? If the answer to that question is “yes”, then that's one glaring reason why a crm is required.
If you have customer details in spreadsheets (or sticky notes), then your sales team — and everyone else who interacts with your customers — is at a disadvantage. When information changes in one system, it isn't automatically updated in the others. There's no complete, single view of every customer’s contact info, orders, and interactions.