@contrapunctus It's usually used as an indicator that if Server A doesn't see the issue with what Server B are posting there's a greater likelihood that if someone starts posting similar stuff on Server A to what you find issue with on Server B then they won't remove the content / suspend the user / moderate it in a way you deem acceptable.
With a lot of the more effective moderation being proactive rather than reactive having evidence that a server you federate with doesn't give a shit about content you don't want to see (For what ever reason) means a lot of folks don't want to deal with the likely future moderation headache of Server A.
But to answer the question I think you're also asking, the only real issue with Server A not suspending Server B is Server B could see replies Server A makes to your servers posts which may or may not be desirable, but again also depends on a load of other factors but for the most part it makes no difference to content visibility.