It allows for the practicality of day trips and similar very short trips.
For example, I live less than an hour from the US border. Pre-covid my day trips across usually lasted 1-2 hours stateside, sometimes even less than an hour. If I have to have test in the US before returning to Canada, I would need to add many hours, possibly an overnight stay to my trip **just for the purposes of the test** and waiting for its results, but that stay will then expose me to a lot more people and situations [hotel, meals, ...]. [Also, the Walgreens in the crossborder town the UPS store is in does not do covid testing, so there is a further drive to a Walgreens that does]. Without the need for a test, I can have a trip that is as short as cross the border, drive to UPS store, go into UPS store to pick up my packages, get back in car and drive back across border. Other than the border guards both ways, the only humans I would interact with or are in proximity with are whoever is in the UPS store at the time, which could be as few as 2 people. Add in testing and an overnight stay and you significantly increase exposure for something that won't tell you anything -- any test taken at that point cannot show infection from exposure that very day, only days before (and likely 5-7 or more days before) in Canada.
I am looking forward to this change, but want to see the actual written explanation/rules from government - and would travel with a printed copy to reference at the border if needed.
Also there is still the question of whether Canada will adjust the advice against Canadians travelling for non-essential reasons, as depending on one's policy travel emergency medical insurance and other
travel insurance may not cover you while that advisory is still in place. Don't want to e.g. fall and break your arm or have a heart attack or be in a car accident stateside on a short (or longer) trip and find out you don't have travel insurance because the advisory is still there and you traveled anyway.
SW