Arch Linux

where does gnome-online-account uses nautilus or viceversa?

I think it's for integration with accessing files on owncloud accounts .

gvfs-goa only depends on gvfs main package and gnome-online-accounts so no additional dependencies.
What difference does it make to you if its files are on your computer or not?

/usr/lib/gvfs/gvfs-goa-volume-monitor only runs if you are mounting a 'cloud' storage account, otherwise it is just sitting there on your computer and not using system resources.

If we wouldn't care what files end up being on our systems, we would choose other distributions. One reason for choosing Arch is to keep your installation clean and small. In this case, however, I was complaining about the fact that one package relies on the other one without any (optional) dependency.

Simple and clean normally implies staying close to upstream (vanilla) and not the opposite including when a piece of software was written with the assumption that a runtime dependency is going to be available.
You can always edit the desc file for gnome-online-accounts and remove the gvfs-goa dependency and uninstall it. This avoids cases where a new user may install gnome-online-accounts, find a non-working portion of the software and assuming something is broken just to find out later that it was deemed optional with no static criteria.
(Please note that the above is not an opinion, but rather an interpretation of Linux software management).
As for my actual opinion, I don't even mind dependencies being completely removed. There, however, needs to be a static criteria as to what is deemed optional or necessary. There is none right now which creates lack of consistency.

Pacman is very versatile in this area. It is mostly why people have stayed at ArchLinux for a decade or more with the spirit that distributions should not be compared or put in competitive situations.