Every project teaches your team something worth keeping
When a project closes, you know which vendor came in late, which one padded the invoice, and which one quietly did exactly what they said they’d do. That judgement is the most valuable thing the project produced, and it’s usually the only thing nobody writes down.
Put it on the vendor’s record and it stops being something you happen to remember. It survives the person who leaves, the year where nobody thinks about it, and the moment you’re choosing again with three names in front of you and no clear sense of which one let you down last time.