Mastering is the process that comes after mixing. It means making sure the production holds when played from different devices and different platforms, and making sure the production is industry-standard and compatible. When you do not master your production it might get rejected by the platform/station, normalised in a very ugly way or feel very quiet compared to the competition.
No, sometimes the director or developer needs us for just music, just design, just ADR or anything beyond and in between. It’s absolutely possible to have us on board with a single or couple of disciplines.