I like the company I'm working at and I'd like to contribute by taking our in-house solutions based on FileMaker to a web application. Furthermore I want those skills as a backup - in case I want (or have) to look for another job, FileMaker isn't going to impress. Also, while the music is certainly my passion, it's unlikely that I (or most other musicians for that matter) could make a living with it.

However those reasons sound rather career- and money-centric. It's not all that rational though - I like making and creating things, whether it's music, software or design - but I value doing things right and with care. Learning a "proper" language/framework and being competent with it would help a lot and I'd consider it as essential.