When is a dispatcher required

[chimichanga]

Sorry if this sounds a bit ridiculous, but when is an aircraft required to be dispatched by a licensed dispatcher. Obviously, I can get in a Cessna 172, file a flight plan and go fly without having another person giving me paperwork telling me that I have been released.

Does the requirement come up with aircraft weight? Type of Flight IFR vs. VFR?

I know in the USA they have Part 91 flights which don't require a dispatcher. I think Part 135 and Part 121 need a dispatcher?

What is the law in Europe under EASA?

Thanks:O