When the camping weekend or vacation is over, people tend to forget about their stuff until the next time. But your sleeping bag is an investment and you want to keep it in condition for the next trip so just stuffing it into a tight place is not a good idea.
Even though the bag may come with a carrying sack, it is not a good idea to keep the sleeping bag in the sack long term. The sleeping bag is filled and if the bag is mushed together for a long time, the filling becomes flat and loses its efficiency.
Also by leaving the sleeping bag mushed together, you are fostering an environment of mildew and bacteria. If the bag was wet or damp when you stuck it into the sack, the fibers will break down.
Lastly, keeping it in the bag will make it stink. It will become musky.
One of the best ideas to keep a sleeping bag in good condition is to air it out in the sun and then fold it loosely and hang it over a hanger in the closet.