It's necessary due to context shuffling. You want to make sure that the plan is splatted into multiple files and then repicked up, analyzed and implemented. ie have the harness create PLAN_{FEATURE_A,FEATURE_B,TESTS,ARCHITECTURE}.md and then pick it up piecewise or in parallel. It's also required if you want to change things between runs, if you try to one shot it you can't easily interrupt an agent.

You don't need plan mode specifically for this, but it amounts to the same thing.

I was tired of the plan mode for same reasons.

you can try matt poccock /grill-me skills for implementation planning. its interviews you to resolve design decisions until you have shared understanding(you and the coding agent).

then it turns it to a PRD,that is my go to right now.

Nah, I just use skills... and started using a custom harness for building context on Issues and milestones created in the git repo. less documents, more goals. https://github.com/OpenZephyr/SilkOps