Not only that they can be up today and down tomorrow, but that you only pay for the time you are using them, which is (relatively) insanely cheap:
www.google.com/search?q=0.34+dollars+per+hour++4+days++1000
www.google.com/search?q=0.34+dollars+per+hour++4+days++1000
But, most workloads are on 50% or more of the time, (see the popularity of amazon's reserved instances) and you save a lot more than 50% owning your own hardware, unless you are paying ridiculous fees for your hardware guy.