The resistor will dissipate power proportional to the power drawn by the fan though, so big fan = resistor is big heating 🥵

Not a big deal for a small fan though.

If you don't put any ohm at all, miss amp just gets pushed all the way through the tube really fast without doing anything. Like if the next part of the tube is an LED, all that amp coming through would make it glow way too bright and then explode or something.

There's a second thing you can do with two ohms. If you line up two ohms one after the other, then a little volt appears in the chamber in the middle between them. But miss middle volt is a smaller version of the big volt, and her size depends on how tight your ohms are relative to each other.

Resistors don't make switches; transistors (among other things) do, though resistors are used as a component part. Long story short: Essentially you don't always (or even most of the time) want as much current as possible; sometimes you want to limit your current and for that you need resistors. This can be used to split a current between two or more wires (the wire with the bigger resistance will get less current and vice versa), or it might just be to prevent a short circuit (too much current will fry or melt whatever you're passing it through because circuitry consumes power as heat), but the point is that the amount of current you want passing through a wire or circuit isn't always "all of it," so you have to use resistors.