Most incline treadmill trainers are more compact than a standard treadmill. They are more box-like in shape whereas most treadmills are a longer rectangular shape. This helps if you have a limited workout space for your machine.
#3 More Joint and Knee-Friendly
Because the belts rise to meet your feet, an incline treadmill trainer is easier on the joints and knees than a treadmill would be. With a treadmill the constant pounding down on the belt can cause a lot of strain and higher impact on your joints. This can contribute to shin splints and knee pain.
#4 Can Also Double As A Treadmill
Some trainers (not all) can do double duty as a treadmill too. So if you just want to walk at a 1-2% incline some days because you're tired, you can. If you want to really burn the calories another day with 30% incline intervals you can. Basically you can use it as a treadmill or as an incline trainer - so you get 2 machines in one.
So those are 4 main benefits of incline trainers vs treadmills. Another thing you should be aware of is that not all trainers are built the same. There are some with 2 smaller treadbelts and others with 1 larger treadbelt (like a treadmill). There are some that give you incline and even decline.
There are some that offer lots of fitness tracking tools and workout "toys" like iPod docks and even a built-in web browser. Others are much more simple with less console options - but more affordable as well. I personally recommend the NordicTrack X9 incline trainer if you're a runner - and the Bowflex TC5 Treadclimber if you're on a budget.
So you have lots of options and it really depends on what you want and need in a fitness machine. Take your time, do some research before you buy and choose a machine you really get excited about using it.