Why is it necessary to drain pus from wounds?

Pus may be infection. Pus is formed when the body is fighting an infection. It may be a sign of a healthy immune system, ergo the term "laudable pus". Pus is full of white blood cells (which fight infection) and microorganisms causing the infection. However, when pus collects in a cavity it forms an abscess. Pus should be drained from wounds to help fight the infection and help the wound to heal normally.
Abscess. The body's defense cells and antibiotics have trouble getting into a pus pocket -abscess. Irrigating and debriding the wound clears out bacteria and exposes the area to healthy tissue with the ability to fight infection. This "evens the odds" in your favor.