Most people decide to buy a touchpad keyboard when they realize how easy it is to mouse with a touch pad surface instead of a traditional mouse device. A touchpad is also much easier for people with disabilities to use as they sometimes have impairments that prevent them from being to use a standard computer mouse. The simple fact that you do not need to apply a lot of pressure to the trackpad surface is usually what attracts these types of users. Most people are familiar with this type of input device from using laptop/notebook computers. These almost always have a built-in touchpad so that if you don't have an external mouse you can still control the cursor. Another reason users flock to this type of keyboard is for the concept of ergonomics. Some say that using such a product instead of traditional mice may reduce finger and hand pain if you use it for many hours every day. Now be careful, because others will say that using a touch pad can make any repetitive stress injuries you may be acquiring feel worse, and nobody wants that. However, you really need to form your own opinion on whether or not they help reduce tension and strain. In addition to those with disabilities or those looking for an ergonomic alternative, some simply use products like this because it just fits better into their workstation setup, and certain programs or software they use frequently may be easier to work with using a touchpad.
These types of keyboards are available in various styles so there is sure to be a model to fit everyone's style and situation. There are some that are shaped just like a standard computer keyboard and have a touchpad surface built-in on the right-hand side (the most common mousing side), others that are considered "mini" in size and may have the trackpad in the middle of the keyboard (which some find annoying and/or unnatural), ergonomic "wave" type keyboards like the B2M-00012 and various wireless models (for those who hate cords or can't accomodate them). So whatever your specific scenario is there is probably a perfect solution out there just waiting for you to find it.
The major manufacturer of computer keyboards with touchpad mouse devices is Adesso. You may not have heard of them, because most of their products are not sold in retail stores, but they have been around for a while and have a huge variety of different styles and shapes to choose from. The only thing to watch out for with Adesso products is that they have a slightly high defective ratio from the factory. This means that you have a higher chance of receiving a keyboard that has something wrong with it than brands like Logitech or Microsoft, but they unfortunately don't make touch pad key boards. In any event, if you do receive a defective product Adesso is pretty good at replacing it with a new one free of charge, but don't be surprised if they slap you with the return shipping charges.