With a plunge router, this workhorse router table will handle just about all of your joinery needs.
The special jig adapts a portable router, preferably a plunge router, for use as a stationary tool. It does not negate the use of the machine in conventional fashion but there are times when accuracy is easier to achieve and it is more convenient to apply the work to the tool instead of the tool to the work.
It's also true that even under normal conditions, router applications require extra attention for support and guidance.
This "master" jig, in a sense, supplies a host of work setups in one compact package.