Let's say you're a musician who sings and plays piano. Your song needs a guitar solo and back-up vocals. Or bass. Or drums- whatever. The problem is, you either don't play these instruments or you don't have access to a studio to record these sounds. No matter how technically inclined you are, there's only so much you can do with samplers- even the best pre-recorded patches lack the punch to really bring a part to life if you really need live players. Getting a band together, co-ordinating schedules, paying for rehearsal space, and auditioning musicians is a costly and time intensive endevour.
Keep the focus on the songwriting and let ProducerPack do the work for you.