Not exactly. You use it to specify which alternative to try first; and the reason it's trying alternatives is so that it can adapt to different platforms: dependency versions (e.g. the old split-base flag), Unix vs. Windows, etc.

Controlling which one is tried first may be necessary to ensure it doesn't accidentally match something that appears to work but is less correct or appropriate than the alternative. For example, using an older library setup may work in installations that have both old and new libraries, but trying the new one first is likely to have e.g. performance or compatibility benefits.