Detailed records must be maintained to satisfy government auditors and as a reference for the company.

Government Auditors
Purchasing records must be available to government auditors to review to ensure that the right amount of tax or levies have been paid.

Company Reference
The purchase order or contract describes warranties and the method the purchaser needs to follow to resolve any problems - i.e. something breaking down while under warranty.

The purchasing records also provide references to making new purchases, especially when searching for reliable suppliers.