Kind of like an automatic transmission for a go-kart. If you just run a clutch, then it's like picking only one gear on a manual shift and being stuck with it. Most torque-converters have as wide a ratio as the old 3-speed automatics you find in cars from the 60s and 70s.... 2.5>1 (low) - 1>1 (high). A TC will allow you great off-the-line starting power and still have a great top-speed, without revving the p_ss out of your motor.
Every kart should have a TC, but not all do, because they're pretty expensive.