Why an LED driver?

I've searched through the posts but can't find an answer (at least a simple one ).

I am replacing some incandescent courtesy lights used in a 24v system. In the housing I have mounted 4 white LEDs (in series) that are rated at 3.7v @ 30ma. Using the LED calculator, I added a 600ohm, 1/2w resistor.

I've tested it and it seems to work fine. Then I thought about adding a variable resistor so I could also dim the lights.

That's when I ran across led dimmers and led driver manufacturer

So here are my questions:

1. Is there a problem powering these LED's with just a simple resistor?
2. Is dimming them with a variable resistor acceptable?
3. What does the "LED driver" give me that a fixed resistor doesn't
4. What does the "LED dimmer" give me that a variable resistor doesn't
5. If I go with an LED driver, will I need one for each fixture or can I drive multiple fixtures with a single driver.

Thanks,