What problems is Casper / highway solving?

Context

Lots of progress in designing efficient partially synchronous Byzantine fault Tolerance (BFT) consensus protocols.

In Classical models finality is binary. Problem because nodes have wrong incentives. Out of many blocks, only 1 is the final one.

In highway model is expressed by fraction of nodes needed to break protocol rules for a block to be reverted.

Flexibility

Nodes can configure their security thresholds, allowing those with lower threshold to reach finality faster than the ones requiring higher levels of confidence.