Lower Costs For Business Supplies, always comparing the cheapest suppliers for anything you need.
B2B discounts and networking with other Amazon Business Users to help you grow
Wider customer base if you have products you can sell through Amazon (only applicable if you sell products)
Amazon Business Account is Free to signup to. This is a Win Win for Amazon and Businesses. Obviously it is in Amazons interest to have businesses buy all their supplies and equipment through the Amazon network. The service offers more than just huge discounts B2B supplies and cut price delivery costs. It will also offer VAT excluded displayed prices, invoices and an account that allows you to track all purchases made by personnel within your company.
Amazon Business account is free, ultimately there are no costs, the benefit comes with the fact that Amazon is huge and has a massive network of businesses, and thus is able to connect everyone with cheapest suppliers and then offer further discounts and savings on larger purchases.