User Stories
Single Developer Project
Every once in a while, Paul gets a random contribution to a project that he works on by himself. Managing those merges is pretty simple, since he only has one branch that he works on. He merely merges the proposed branches in as he works on other issues. He does not need a merge queue at all.
Small development Project
Tarmac has a few main contributors. The development focus branch is owned by a team, but no one commits directly to it. It's contributors have rules for reviews and landings.
Bazaar
Bazaar has many contributions every day. It is simply unrealistic to expect someone to manage the merging of all of these branches and still maintain quality.