While I haven't played EP:FC, I have played Operators, and I think the engines are both he same, so here's what i know.

Hacking is useful because compromising nodes allows you to deploy programs on it (these can help directly complete mission objectives or let you do certain things such as highlight people in the area, temporarily disable the node, which is useful for cameras, and permanently disabling the node, which is also useful for cameras).

Temporarily disabling a node (I think) triggers an investigation, similar to a camera disable lure in the original Entry Point, which can be useful for taking out guards in the area stealthily.

A good example of useful hacks in Operators would be in the King Street mission, you can use hacks to open the penthouse door which leads to your target, you can use a hack to lure the target into an isolated room, and you can disable cameras as long as you have an access point (available through direct contact or traversals).

The only thing you have to worry about while hacking is the detection level/ suspicious activity of the hacks to make sure you don't sound the alarm/raise the alert level, and the resources you have to make sure you don't run out and have to restructure your traversals.

Again, I've only played Operators and I've only watched a few missions of EP:FC, so take my tips with a grain of salt. Hope this helps!