Theres no question it retains freshness. As long as you use it properly. Theres no comparison between a properly vacuum sealed piece of meat and say something in a ziplock bag.
I wouldnt live without one.
All I have is anecdotal evidence but I can say with confidence that it will increase the amount of time it stays fresh in almost all cases. The ELI5 explanation is that air and bacteria will cause food to spoil. Less air and bacteria mean less spoilage. Curries and stews would be perfect for this as long the sealer you have is adjustable for wet or ‘moist’ foods. .
Some foods don’t seal well based on their shape and won’t allow the sealer to get all of the air out. Like big pieces of broccoli or too many things stacked in one bag. In most cases there are strategies for getting things more aerodynamically shaped in the bag.
I hope this helps!