A midi controller keyboard does not have a sound generating module in it and is used only for sending midi messages to external software / hardware. So how is it useful if it cannot generate sounds on its own? It is useful if you have a predominantly software based recording setup. If you use a software sequencer for sequencing the tracks, Controladores MIDI and if all your instrument sounds are generated using soft-synths and VSTs, then you only need a midi keyboard controller. You just have to connect the keyboard controller to a computer or a laptop, which will have all the necessary software programs
There is no need for you to go in for a full fledged digital keyboard. A midi keyboard controller will be able to send all the performance related messages to the software programs. The controller keyboard will send midi messages to capture all of the performance related parameters. It can send messages which indicate which note is to be played, how hard or soft it is to be played, any effect like pitch bend or modulation was used, etc. Some keyboard models will also have knobs, and sliders in addition to pitch bend and modulation wheels, so that you can easily add effects.
You will find controller keyboards that have less than 61 keys as well. Many users actually record one track at a time, so they really do not need sixty-one keys since they will not use both the hands together. Besides, a smaller keyboard will take less space as well. A smaller keyboard will help you easily demonstrate your recording capability to others. Whenever required, just dump your laptop, soundcard and the MIDI keyboard controller in a bag and carry your portable recording studio with you.