They are not always the answer. For instance if you need to isolate a section of pipe but you don't want to cut into the line, a Jet Swett will not work for you.
We have some systems that purposefully do not have isolation valves so that they can not be turned off accidentally or oterwise. When repairs have to be made to the equipment they service and the common header can not be shut down the only way is to freeze the line and maintain it for a few hours (maybe 8 or more) while repairs are completed.
They go to 4". The bigger sizes have a flow through valve which allows you to hook up a hose and bleed pressure off.