Because they always surprise you.
Whenever I start a new job, I speak to whoever has hired me, normally higher management who give me an idea of the project, why they need me and where they think the project will go. Then, after a little settling down I'll get to speak to some real users, either external customers or internal system users and generally hear things completely opposed to what management have had in mind.
If it's an existing product, the users will already have a stack of work arounds and ideas they have to use or would like to see in any improvements. Often, these are work arounds to continue doing something which has been specifically forbidden by processes or higher ups.
For new systems, users will have a different idea of what "task complete" looks like than the original plan, they'll always want more information, or less, or have it phrased in a different way.
Essentially I'm saying the best part of my job is talking to people and really listening, the knowledge to build the right solution is already there, it just needs extracting.