As stated by @pradkm RTG is done only on 1 side of a connection, typically either Access or Distribution. Also as @pradkm said, RTG is an easy option to use instead of STP.
Biggest advatanges of RTG are easy to config/use vs STP, and Juniper can now interact with any other vendor switch/device as everything happens on the Juniper switch.
If you understand active-passive dual NICs on Servers, RTG is basically the same, except on various Juniper switch platforms.
FYI only.