This is no longer true, at least since 2003 in the US. Car seats are now attached using the far-more-secure three-point LATCH system. I never once “used this feature” when installing a car seat for my now-twelve-year-old.

The LATCH system has a maximum combined weight of the carseat plus child of 65 pounds. As a result, the threshold varies with the weight of your carseat and weight of your child, but a typical 4- or 5-year old probably should be using the belt.

https://csftl.org/whats-the-deal-with-weight-limits/

Personally, I don't think that was a good threshold to set. I agree that the LATCH anchors are better. In 2003, I suppose people were mostly using carseats for infants and toddlers in little 10-pound styrofoam shells, now we know kids are safer in carseats until they're significantly older and heavier, and they're in 25-pound adjustable, heavily bolstered bucket seat style carseats.

For what it's worth, whilst my car and carseat have the LATCH system, this particular carseat requires use of the seat belt instead of the LATCH lower anchors for kids over 30 pounds.

yeah, I had no idea this was the purpose behind the feature until this thread - my kids are young enough that our cars have always had LATCH since they were born.