Standard practice. No statute requires it and no insurer demands it of them. It is what a homeowner should insist on, because without it a mistake is a lawsuit rather than a claim.
Ask before you hire
If any of this is hardwired, what is your ECRA/ESA licence number?
Is the monitoring contract separate, how long is the term, and what does cancellation cost?
Do I own the equipment outright at the end?
Does a monitored system reduce my insurance premium, and by how much?
The equipment is often cheap because the money is in a five year monitoring contract. Read the term.
You asked for the trade Security and alarms, and the words "storage between closings", and businesses serving Mississauga. Of the 12 businesses listed, none match every one of those.
The cost band and the licence requirements above still apply. They are what you need to judge a quote from anywhere else: whether the price is in the range, and whether the person quoting is allowed to do the work at all.
Listings here are seed data for demonstration and are labelled as such on each profile. Cost bands are what GTA homeowners are typically quoted, not offers. Always confirm a licence number against the issuing registry yourself before work starts.