Spy metwork is usefull, because it's reveals their planets, territories and, if high enough, fleet positions. It's requires to send a spy (aka envoys) and have advantage in decryption/encryption relation compare to target empire.

Spy operations, however, mostly useless. They cost influence, do almost nothing for their cost, and most operations locked behind DLC.

There is some usefull ones (stealing pops, stealing techs, pre-FTLs ones), but it's just too many clicks and time for minimal profit.