The term 'return tracker activation' usually relates to the process of submitting your Passport to IRCC from within Canada.
When you submit your Passport to CPC from within Canada, you have an option of adding an empty (prepaid) Canada Post Express / Priority service envelope within the one you send your passport in. The second envelope carries as tracking number.
The moment CPC submits your passport to Canada Post for delivery (obviously after processing it), the tracking number becomes active and you can track its status. So you have to keep 'tracking' the number on the Canada Post website to know when it becomes 'active'.
Thus, the benefits are
1. You receive your passport sooner because its Express/ Priority service
2. It enables you to know where your passport exactly is on its return journey to you.
3. Its more secure because you can choose the option of a 'signature on delivery'.
TLDR version: So if you see someone on MyImmitracker say 'return tracker activated', you know that their application status is complete and only the return delivery of their passports is pending.