On Ubuntu 18.04 (or 20.04, etc) will install recommended packages along with apt-get install, I understand that depended packages should be installed, but why recommended? who decides the recommendation?
But I guess there will be a valid reason, what are the reasons?
There is no particular reason except, likely, to improve the experience of the average Ubuntu user. Installing both the app with the recommended packages immediately ensures that all the functionality is there out of the box. The user later does not have to discover missing something, that would require an additional, not essential package.
The more advanced user that wishes tighter control on what is being installed may create an alias to apt that includes the --no-install-recommends option, or change the setting system wide.
The person doing the packaging decides what is required, recommended, or suggested.
That person might be at Debian or might be at Ubuntu.
There are a number of guidelines in the Debian Packaging Guide to determine dependencies.
Oversimplifying the logic to clarify the reasoning: Those guidelines help the packager determine whether or not excluding the dependency will break the software entirely ("depends"), or merely break major features of the software ("recommends").
