High speed spindles are essential* when you're working with small cutters, and soft materials.

A conventional mill is generally aimed at being able to machine steel, so you need pretty low spindle speeds, and suitable torque to handle large diameter cutters at low speeds, which doesn't work well with small diameter cutters and materials like aluminium.

If you want to see the benefits/limitations of spindles, just run some figures through a speeds and feeds calculator.


*I say essential, but you can use small cutters at slow speed, it's just that it takes quite a while to the limited chip load, and subsequent restriction on feed speeds.