A short range missile (such as GMLRS, which HIMARS also fires) does not have the glide kit. This means the missile is much faster, but for maximum range the missile has to follow a (vaguely) ballistic trajectory. With the glide kit, it can trade much of that speed into additional range by gliding to the target. This means GLSDB gliders can hit targets at further range, but being slower, it’s easier to shoot down and makes it less able to hit extremely time sensitive targets. IIRC, GMLRS missiles have a time-to-target of 2-3 min from launch, while GLSDB gliders have (a guesstimate on my part) more like 10-15min from launch. GLSDB gliders can also sacrifice some of the range to loop around and hit targets from different angles, while GMLRS missiles are pretty much only top-down. GLSDB gliders are also significantly cheaper than GMLRS missiles.