Very confused about site link bridges

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I am very confused about the benefit of site link bridges and when we should create them. As far as I know, they are used IF the network is not full routed (which I am not sure what does it mean, what is a fully routed network ?). In this case, we disable site link bridging by removing "Bridge all site links" check box. Does a site link bridge represent a physical connection such as a site link or not?

The book I use says that we use them to connect site that are not directly connected and site link transitivity is disabled (that's the same as removing "Bridge all site links" check box). And it says that we do that to control replication between sites which is completely vague. What I have got so far is that we can control replication between sites by configuring site link attributes replication interval, replication schedule, and cost. Those can be configured easily by going to site link properties.