The macOS app version of Codex I have doesn't show reasoning summaries, just simply 'Thinking'.

Reasoning deltas add additional traffic, especially if running many subagents etc. So on large scale, those deltas maybe are just dropped somewhere.

Saying that, sometimes the GPT reasoning summary is funny to read, in particular when it's working through a large task.

Also, the summaries can reveal real issues with logic in prompts and tool descriptions+configuration, so it allowing debugging.

i.e. "User asked me to do X, system instructions say do Y, tool says Z which is different to what everyone else wants. I am rather confused here! Lets just assume..."

It has previously allowed me to adjust prompts, etc.