[XCSme]

Not only performance optimizations, but also UI/UX.

If you ask to implement a custom dropdown that does something, it often comes with good spacing, aria-accessible tags, keyboard accessibility, etc. A junior dev wouldn't think of all of those.

Also, it will probably choose the right HTML elements to use for it (i.e. maybe the modern native popover functionality instead of implementing it with custom JS, which would indeed be more efficient and less code).

I am not saying it would add caching by default (even though, it might suggest that), but it's more likely to choose whatever the best options are and to use them as they should, including going around knowing limitations and gotchas.