There are so many wonderful music software tools are available with the wave of a credit card, why would you want to roll your own? For one thing, you may want to do esoteric things that no commercial software will do. With the aid of a program like Pd, you can generate and process complex streams of MIDI and audio data for a live performance, or build your own software-based synthesizer and effects. Beyond that, solving technical problems in software design can be a fascinating intellectual challenge. You get a sense of accomplishment from creating something that works exactly the way you envisioned. Then there's the money factor.
