Ideal blockchain utilization


in the past weeks I hear from several sources, that the ideal utilization of Cardano is 100%.
I was wondering why this is, because normally one would prefer to have reserves in case of peaks.
With Ouroboros this seems not to be the case because of Backpressure.
But as I understand Backpressure it only delays transactions to be able to handle the load.

So can anyone explain why 100% and how Backpressure solves the problem?