A shop switcher is a switching locomotive employed at a railroad's maintenance workshops to move around locomotives undergoing maintenance and therefore not themselves operable. Even if the locomotive is actually in a functionally operable condition, it might not be desirable to start it up in order to move it under its own power, especially if it is a steam locomotive (in which case starting it up from cold may take 6 hours or more), while an electric locomotive may not be under catenary while in the shops and therefore can't obtain the needed current.
A shop switcher does not need to be very powerful, for all it has to be able to pull is a couple of dead locomotives or a short string of cars around a fairly small facility, and involving no main line travel. Its top speed also does not need to be high. The most important characteristic of a good shop switcher is that it must be short in length. The reason for this is that it will often be required to fit on a device such as a turntable or transfer table along with the dead locomotive it is moving. Such devices are quite restricted in length. For this reason, shop switchers are normally tank locomotives if steam powered. The second most important characteristic of a shop switcher is that it must be easy to keep running all day with minimum attention, since it will not be in constant use but may be required on a moment's notice. For this reason, a diesel locomotive is much preferable to a steam locomotive and for this reason many railroads began using diesel locomotives for this purpose. The only reason some railroads didn't is that buying a new locomotive for these purposes wasn't often done; they were more often locomotives retired from other work and rebuilt for this task.