Virtualization is a very broad topic. Give a look at WhatIs, you will find some good points. I can tell you why I use VMWare. With VMWare I can test products in a protected environment. I run the virtual machine and if something goes wrong, I just delete the image and start a new one.
There are lots of software companies that offer VMWare images to test the product. You don’t need to install and configure the software. You just get the image and start using.
I’ve also seen education companies that configure VMWare images for their courses. You just do it once, copy it to the student machine and it’s done.
Just my 2cents!
I prefer VMware because it is a more mature product which I feel is more ready for Enterprise use that Hyper-V. For example VMware allows you to cluster several host machines together using shared storage (such as a SAN) which allows you to instantly move a guest machine from one host to another with no downtime. VMware also have some excellent auto restart features so that in the event of a host machine failure the VMware system will auto restart the machines which the now dead host was running on other machines in the cluster.
VMware 3.5 now gives us the ability to move a virtual machine from one drive to another on the host without ever taking the machine offline. So if your disk starts getting full, or having a performance problem you can move machines off on that disk with no downtime.