^^this. The truss rod in the neck is (supposed to be, at least) adjusted to provide a slight relief in the neck that allows the strings to ring out properly. If the strings are not kept properly tensioned (tuned), you will either get a bow (less common, would be strings increasing in tension somehow on their own) or a back bow where the lower string tension causes the truss rod to bend the neck the other way. In the short term, this might not cause permanent damage to the neck, but left this way, when it does require adjustment it is possible to stress the neck itself and crack it, or possibly delaminate the fretboard At that point, unless it’s a valuable or sentimental guitar, the cost to fix by a good luthier just isn’t worth it.