The main advantage and reason for using those bands is lack of available frequencies on other bands in many areas.
806-896 MHz was taken away from TV broadcasting in the late 1970s and 746-806 MHz was taken from TV broadcasting beginning around 12 years ago.
The only real technical advantage is the way the 700/800 MHz band usage was planned with a large tx/rx offset separation; base/repeater outputs at one end of the band and mobile/repeater inputs at the other end. On 700 MHz base transmit power and antenna patterns are required to minimize coverage outside of the desired geographical area.
The disadvantages are the higher the frequency the higher the path loss, transmission line loss, and loss in other components, more rapid and sharper fading, and more multipath interference because stronger reflections occur from smaller conductive objects, which in some cases can help because signals can be reflected into and between structures more than lower frequencies.
In cellular type systems the disadvantages can be an advantage because range is not as great and when you run lower power you can resuse frequencies with smaller distances in between stations on the same channel than would be possible on lower frequencies.
