In certain situations, it's possible to put a new roof on an old one, and, in some cases, it can simplify the process. For example, with a shingled roof, installation over an existing roof might be easier than tearing off the old roof, since you can use the old shingles as a map for laying down the new ones.
Tearing off an old roof costs money, so you may be able to incur financial savings by skipping this step and installing a new roof over the old one. However, it is important to determine if the building can handle the weight of two layers (or more) of roofing. Local building codes also need to be considered, since some communities prohibit residents from installing new roofs over old ones.