CommandUI needs to load data via the REST API for some search "scopes" such as post types, users, etc.
By default, it preloads some data, but only until you once open CUI for the first time during a page load.
If you enable "Seed Dynamic Actions on Page Load", the preloading starts immediately after the HTML page is fully loaded, thus everything being even faster.
The "downside" is that you perform a couple additional API requests on each page load.
I personally don't care about that on my sites. A second of my time is worth 1000x a second of compute and since CommandUI is never loaded for visitors, this does not matter unless your server is super slow or overloaded.
Since many WP users run on those slow and overloaded servers, this setting is opt-in.
Hope that makes sense.