I never really thought about it, but, looking them up it looks like they're basically the equivalent of the FBI(+ATF), but also sometimes are contracted for local policing by towns too small to maintain their own department? Is that accurate?
But the US have a lot of specialized police agencies -- the RCMP is also the DEA, the Secret Service, the US Marshals... probably many more I'm forgetting or don't even know about.
* CSIS is a civilian national intelligence service
* RCMP is a national police service
They're related because CSIS was born out of recommendations from the McDonald Commission. After the FLQ crisis, Canada understandably had a bit of a 9/11-style overreaction, and the RCMP Security Services got involved in a number of scandals stemming from overzealous and illegal activities. The RCMP Security Services were consequently abolished and their responsibilities were given to the newly formed CSIS, which was overseen by the newly formed SIRC. One could consequently reasonably say that CSIS was the successor to the RCMP Security Services.
However, it's really impractical for policing to be completely divorced from intelligence work, so much as the FBI does intelligence work despite the US having numerous civilian & military intelligence organizations, the RCMP continue to do intelligence work.
I grew up in Canada and read the McDonald Commission's report (well some of it), and was there when CSIS was created. Saying it's the "successor" to the RCMP is like saying Canada is the "successor" to Britain.
These postings often require the new recruit to move - not just towns - but whole provinces away from their extended families. Throw in the cold, boring nature of these postings and what you get is a very bitter officer. One who is looking to pad their resume and move up the ranks and get out.
It's also common for officers to be internally disciplined in this way; The best cops get the most prestigious postings, and the worst get the opposite (just like Catholic priests).
(To be clear, I have no idea if this is true in this case - it's more of a generalization)
Think of them as the “default” service.
In many provinces, they’re the primary police service for all towns/cities except the largest ones.
Other provinces have a provincial police service to be the default in towns/cities that don’t have their own municipal police service.
Consider that the officers posted to a remote reserve like Duncan's First Nation might not be the best and brightest of the service. More Keystone Kops than Dudley Do-right.