At any one time, at least 10% -> 30% of your desktop fleet is on the move.  Why do we care?  Here’s a couple of reasons why:

You will generally be billing the customer a subscription rate of some sort.
If you don’t know what’s out there, you could be missing revenue!
Let’s look at the mythical example of ServiceProviderCorp:

PC’s under management 1000
Revenue per month per PC $25
Percentage of PC’s not being billed for 10%
Loss revenue per month $2,500
Loss revenue per year $30,000

That’s right, $30,000 per year.  Do the math yourself on your desktop fleet.  You might think, “oh, it’s only 10%”, but it REALLY ADDS UP.  I’d really recommend that you do the math yourself.  You might be thinking, “oh, we’re good at 5%”.
With a 2000 seat customer, that’s $30,000!  And then there’s the whole “We’re not paying that bill!”

“Not paying that bill!”
What are billing disputes costing your company to resolve?  Both in turns of $$$ and in turns of customer satisfaction.  You DO NOT want the situation where the customer has no confidence in your bill.  Because then it’s a short jump to the customer losing confidence in everything else you do.

The customer will not accept that a PC you are responsible for, has caused a security breach, because your asset system cannot see it.  It’s YOUR responsibility.  THAT will be the customer’s point of view after a virus outbreak.

It happens.  A good desktop management system (DMS) can provide reports on memory and hard disk size changes.  If you have a 2GB system, and it’s memory suddenly shrinks to 512mb, it means something has changed.  And that something is generally THEFT.  I’ve seen it happen.  “But Dale, we have 10,000 desktops to look though”.  Manage by exception!  Automate it!  Have the DMS only highlight differences to you.  You could have the DMS email you and the responsible customer manager.

A Desktop Management System is general a suite of products which cover Asset Management, Software Deployment and Remote Desktop Takeover.  Some examples are the Microsoft SMS 2003/SCCM 2007 & Computer Associates Unicentre toolsets.  There are others.