The carburetor has a high speed circuit (main needle) and an idle circuit (idle needle). They need to be adjusted separately because they control the fuel under two very different conditions. Both come into play at part throttle. This is similar to most other small engines such as chain saws, weed whackers, a lot of mowers and so on. It's also similar to carburetors used on cars & trucks except those usually had a fixed jet or some sort of metering rod/jet system for the high speed.

I've seen a couple model engines that were throttled but had simple carbs with no low-speed needle. They were both fairly small, .074's I think. The trade-off is a less reliable idle.

Having two screws to adjust is no big deal. Just follow the instructions and it'll go well.