Originally Posted by dabac

It's common for after market threaded forks to be sold at raw length, ie with a long threaded section. The extra cost of cutting a longer thread is compensated by not having to stock several versions of the same fork in order to match frames with different length head tubes.

Usually people will do a test assembly, mark up the excess, pull the fork out, thread the lockring/cone down below the mark, then cut the excess off.
Backing out the lockring/cone will then help to clean up the threads, and reassembly will give you a clean, factory look to the install.
Some people can't be bothered, and depending on design(some lockrings allow the steerer to come clean through, others will bottom out) will either slap on a stack of spacers until the lockring tightens up, or simply keep screwing down on the lockring, leaving a length of threaded steerer exposed.

As Andrew writes, there's a point to having the wedge of a quill stem(adapter) engage below the threaded part of the steerer.