MIDI keyboards are used for triggering/accessing any sound that can be recorded, sampled, or synthesized. For example, my MIDI keyboard is just a "controller." It doesn't produce any sounds of its own. Instead I plug it into my computer soundcard and use it to play soundfonts (recorded samples). This way I can add drums, strings, brass, scratches, or any sound, etc. to my arrangements.
You can use a MIDI keyboard to access/trigger the sounds of any hardware or software synth or sampler. They are very powerful, inexpensive home studio tools compared to hiring a brass section and studio time everytime you want to add some brass flavor to one of your recordings.