This allows you to call up, say, program 17 and have some confidence that it will do the job musically, because program 17 will always be a drawbar organ sound on any General MIDI synth. But the basic principle is still true. The transmitting synth has no idea whether the receiving synth is a General MIDI instrument or not. The program change message is always the same, but if the receiving synth is one you haven't worked with before, sending program changes will give you more musically predictable results if it's General MIDI compatible than if it's not. Pressure messages are used to alter the sound of notes while the notes are sustaining.
Pressure comes in two flavors.
Pressure is also called aftertouch. Continuous controller messages make other kinds of changes in sustaining notes. MIDI has room for more than 100 continuous controllers to operate on each channel, but very few synths can respond to this many at any given time. The continuous controllers are numbered. Some of the numbers have well-defined meanings, while others are left undefined so that each instrument can use them in its own musical ways.