Network-wide suspension is an expedient way to suspend a user network-wide, covering even sites they may not have joined yet. That's not frequently useful, which is why it isn't frequently used... But in those rare situations where someone is going out of their way to spam / troll / harass multiple sites in a short period of time it is a handy tool.
I start with that description because you need to understand: network-wide suspension is not a necessary escalation from per-site suspension. That is, one might conceivably be suspended numerous times on a single site without ever being suspended anywhere else; one might even see their account deleted on one site without that resulting in consequences elsewhere. Network-suspension is a specialized tool for a small set of relatively specialized circumstances; in some ways it is more limited than normal per-site suspensions. For example, there's no corresponding network-wide "unsuspend".
With that in mind, please consider this hypothetical scenario:
- User acts up on one site, gets a short suspension
- User flies into a blind rage, hits numerous other sites to post rants against whomever they blame for the consequences of their actions in #1
- Employee suspends user network-wide to avoid having to spend all evening chasing them across the network.
Now, at this point the user hasn't actually caused problems on most sites, and even on the site where they originally acted up there's some possibility that they would be able to resolve the situation once they've calmed down and can have a sensible discussion with the mod team. Which is to say, it's completely reasonable that they might be able to negotiate a reduced suspension on one site or another - after all, they could always be suspended again if they go back to causing trouble.
But talking an employee into manually unsuspending them on 170+ sites is another matter. That's a fair bit of extra work to solve a problem that the user should've known better than to cause in the first place. Said employee is much more likely to advise them to either wait it out, or negotiate on a site-by-site basis.