But a process run in a systemd session still can access all the device nodes under
/dev, so what really is the point of systemd introducing the concept "seat".
Are you thinking in terms of running as root? This is essentially for non-root users.
The concept "seat" is for situations where you want to service a maximum number of local users with a minimum amount of hardware (e.g. for schools or similar).
Computers can have multiple displays, keyboards and mice connected to a single desktop box, so with systemd, one desktop with two displays, keyboards and mice can provide two separate GUI sessions simultaneously if desired.
In a normal single-seat configuration, any hotpluggable USB devices normally have their device node permissions set so that a locally-logged-in user can automatically use them, but users logging in remotely (e.g. with SSH) cannot use them unless they are root or members of special user groups like
With a multi-seat configuration, any such devices will by default belong to the default seat
seat0: the administrator can configure specific devices to other seats instead.