That’s a function of the Wake-on-Lan (WoL) function. I recall some discussion of this elsewhere on the forum and recall (perhaps incorrectly) that Roon does not have a WoL function built in. I think folks add a separate app to their tablet or smartphone that sends the “magic packet” to WoL their computer, and then afterwards can use the Roon remote app.
p.s. Other music servers I’ve used do have a built in WoL function. For example, LMS.