Summary: to give a project the best chance of success (good work, happy users), we need frequent access to the users of the software. Without it, we run the risk of making incorrect assumptions and spending time and money building something that doesn't solve their problems.
When we make software there are two things that we try and keep in mind: build the thing right and build the right thing.
By build the thing right we mean use development best practices (often following agile ways of doing things, using Test-Drive Development, writing clean code, doing code reviews).
By build the right thing we mean doing User Experience stuff, usually with a User-Centered Design flavour.