If there isn't dependency caching, then we run into all sorts of rate limits, errors, etc. Also, it cuts down on a huge source of flakiness (I'm looking at you npm!).
Topic on Talk:Wikimedia Release Engineering Team/CI Futures WG/Requirements
Don't forget how often Composer has flaked out due to rate limiting from GitHub's side (phab:T106452)!
There is a requirement "Must support dependency caching – we have castor, maybe we could do better? Maybe some CI systems have this figured out?", which is meant to cover this. The wording could certainly be improved. Suggestions?
When I initially wrote the comment, it was listed in the "would be nice to have" category - but you moved it a short while after, so all good here.