Why Use QTP?

It's the de facto standard

If you look in the QaForums, you'll see that the forum on QTP has over 20,000 threads with over 97,000 posts. The next most popular automation tool (ignoring WinRunner) is Borland SilkTest with around half as many.

This means that if you have a problem:

 1) Chances are that somebody has already had that problem (or something similar) and solved it.

 2) You can post a problem and within a day or so, around 200+ people will have seen your problem and possibly solved it for you.

 3) Third-party add-on software companies are more likely to produce a testing add-on for QTP than for any other test product.

 4) If you need more staff, you'll have more to choose from.

It uses VbScript

This has several advantages...

 1) Lots of people are familiar with it so you can get staff more easily and they're easier to cross-train.

 2) There's a huge existing code base available (usually for free) for doing everything conceivable.

It's expensive

From a testing point of view, this indicates your company has some commitment to Testing - they're not trying to do it 'on the cheap' or as a second thought.

 1) They understand the importance of testing.

 2) They value their test personnel more highly.

 3) It satisfies the Joel Test

It gets the job done right out of the box

Some automated testing tools require endless configuring to get into a state where they can get anything done at all, they often require a range of other add-ons to be located, installed and configured.