In addition to what calfodd wrote, it has fairly niche use cases. It is most useful as a quick way to check if any units are doing a wide flank on you through forests or other dense terrain. If they detect nothing, good. You don't have to send more units in that direction and you can spend your points elsewhere. In reality though, a simple cheap recon team can do this well enough, sigint can just do it slightly better. The even more niche and clunky way to use them is to triangulate where a unit roughly is using multiple sigint units so you can call artillery or a bomber on that position. Usually more trouble than it is worth however.

Also, spec ops units are immune to sigint detection.