It’s the only way to take full advantage of many software programs

If you think a keyboard is only for playing notes, four or five octaves may be sufficient. However, many virtual instruments (e.g., FXpansion Geist, Native Instruments Kontakt, IK Multimedia SampleTank, EastWest’s Play engine, etc.) use MIDI keys not only to play specific notes but also to trigger articulations or variations on a basic sound. 

If your main USB MIDI controller doesn’t have enough notes, no worries—trade it in for that deluxe 88-note weighted keyboard you’ve always wanted (hey, you only live once). But if you lack the space or finances, add a second USB MIDI controller for doing switching—even if it’s just something like a little Korg plastic keyboard designed for mobile applications. Your sequencer probably won’t be able to merge incoming MIDI streams, but no worries there either: MIDI Solutions’s Merger, which costs about $70, will merge two data streams to a single output. There are also several DIY circuits for MIDI mergers on the web.