Not always true. It depends on car seat, really.

We have 3 kids, youngest one almost 2yrs old and rides in this Chicco front-facing seat in the middle.

The "base" (not detachable from car seat) is secured with middle seat-belt. The seat itself is secured by running the dedicated belt under headrest and over the seat back and latching to the middle anchor point in this picture
.

Just having the car seat secured with a belt will still allow too much forward movement in case of front impact - a tip over of sorts. Having the dedicated belt latch will keep the car seat from tipping over like that.

On the other hand, the dedicated latch belt should not be used all by itself, since the base needs to be secured as well to the bottom.