I have a lumbar back herniated disk. In what way would a course of physiotherapy help me?

Reduce pain. The therapist can show you ways to strengthen your core (back and tummy) muscles, improve flexibility, work on leg strength, improve body mechanics (how you move and lift), and overall fitness. This may be enough to get you over the disk herniation.
Reduce symptoms. The theraputic exercises can help wiggle the pinched nerve free from the disc that is pinching it. The theraputic massage can reduce muscle spasm and pain. Then exercises can strengthen your back/core and possibly reduce the incidence of it coming back.