1. Get it right the first time

If you don't you're in for a real treat. Most likely you'll have to redo certain areas of the application or quite possibly the whole entire application.  I have been in several situations where the previous developer didn't analyze the customers business in depth and whatever comes out of the development process does NOT get the job done for the customer.  What does that mean?  An application that has to be re-created or go through a major overhaul.  NOTE: This is very expensive.

2. Identify opportunities

To continue on from the first point, if you map out how your users currently work, there is no doubt you will find opportunities for how they could improve their process.  This is what users cannot articulate and what technologists can.  The best part is, some of these opportunities maybe much easier to implement but could be highly appreciated by users. By finding out how the user's work, you can also remove some of the painstaking features that the users were asking in the first place.

3. Increase user base faster

Making your users happy naturally makes them want to spread its usefulness by word of mouth.  But not only that, if you know how they currently work, it makes it much easier to communicate to your target audience why your application is worth using to them.  You don't need a hard sell.

4. Developers get on board faster

Like the old saying goes, a picture paints a thousand words.  Even among people who love and create technology, most all would prefer to see a diagram of how users work rather than read a 10 page specification document.

5. Developers are much more effective

Not only do developers get on board faster, they understand how certain features that they are developing will help the user's process.  The reason they are developing a feature is not because "AJAX is hot and every new app is using it!"  The reason is: "This is how our target audience currently works and this is how they'll be far more effective with our product.