Why is project closure important?

Question

I know that every project needs to be closed, but why do many references stress the importance of project closure? It's not like it's the most important phase in the project. Even if the project closure is done very loosely (just making the client sign on the acceptance and handling the formalities), the project would still be OK as it's officially done!

Can someone convince me on why it is THAT important?

Answer 1

There are several very important activities that are a part of closure.  These include:

financial closure: all bills paid and revenues received
keeping of historical records: this is important to future project success
lessons learned: also important to future project success

By keeping good records of past estimates, plans, and other project history, you will be  gathering information useful for other projects.  I've often had team members say "I don't know how to estimate that" and then referenced data from a past project.