For quite a few products, releasing takes time. QA, auditing the security, localizing, generating the builds, executing exhaustive in-depth unit tests, flushing caches, writing release notes, publicating on different websites... it can take a while!

When the cost of the release goes above a certain amount, due the sheer number of features or the thoroughness of the release process, it starts to make sense to mutualize release tasks between a bunch of features.

Bundling those features on a time basis is quite often a local optimum between an acceptable delay for the product team and an acceptable cost for ops/release/qa/...