Mosfet lamp dimmer

Why the MOSFET? It seems to work much better without it.

It's a valid point. If we are going to use a Mosfet or bjt etc then it needs to have a purpose....That is often for when a low voltage or current is needed to operate a large voltage or current. Another purpose is where it is used to keep things switched on or off instead of using a relay, and yet another purpose is for amplification....but there is no benefit in putting it in a circuit where the same result can be achieved without one. A Mosfet is generally better for full switching straight on/off since they are prone to unwanted heating if used otherwise....there are exceptions though, and exploring possibilities is a good thing.

Loh

Also, it works better with a regular (bipolar junction) transistor.