Why does spacing work, in terms of storage and retrieval strength?

A gap lets retrieval strength drop, so the next review becomes an effortful retrieval — which is exactly what adds the most storage strength.

Spacing and the storage/retrieval theory are two views of the same mechanism. When you leave time between sessions, retrieval strength decays; the next time you study, you have to work to recall the material, and that successful effortful retrieval banks durable storage strength. Cramming skips the decay, so every repetition is easy and hollow. In short: forgetting a little between sessions is what makes the next session count.