Or at a pump. Often you will use a spool pice at equipment that requires frequent service. This makes it easier to remove the equipment - and also to replace with another model/brand since you only have to modify the spool piece.
In addition to the above situations, a removable spool is preferred for equipment isolation for safe entry. A 3 foot spool piece at a vessle nozzle is convenient because you simply remove all but one of the lower bolts and swing the spool down. That connection is now isolated without needing a blind. A less convenient but still preferred alternative is if you have an elbow immediately off the nozzle, put another flange pair on the vertical run and a lifting lug on the elbow for rigging out the piece.
The most frequent application I have seen is as pe MortenA- at frequently serviced equipment pumps and sometimes valves. The requirement for a removable spool at a pump depends on the size and type- with some pumps it will be almost mandatory to have one or you won't be able to unbolt the casing.
Another application is for clearing of bogged of process lines- particularly high density/coarse applications. Generally it is best to have these on elbows to easy access to high pressure water cleaning in two directions (you can achieve the same result however with a reasonable size stub and flange on the elbow).
