Managing assets should be more than just an "IT" static assets data. It also should include financial data and fields for data integration like: Invoice number (of the purchase), invoice date, supplier. Supplier contact details in case of support is required etc. Generally suppliers and contact list is handy.

Work on financial data for assets has already started and you can expect it soon. However, as a workaround till we ship it, you can make use of custom fields. 

Admin --> Configuration Item Types. Click edit against whichever category of CIs you want to add custom fields to. Drag and drop the required fields.

Hope this helps!