Your jetpack troops are combat engineers (let's face it, jetpacks are temperamental things and it takes technical know-how to keep them working in the field) and their duties are the traditional duties of engineers: establish routes that other troops will use to follow them, sabotage enemy routes and fortifications, and clear out any obstacles that would stop the rest of the army.
For pioneering work, jetpack troops are ideal for establishing a beachhead. Whether it's literally storming a beach and taking gun emplacements on the sea cliff, jetting over a river to surprise dug-in enemies, or bypassing an enemy-held bridge, they are invaluable for securing an area long enough for the rest of the army to catch up. Even if you're not under fire, being able to jump over rivers and up cliffs is a huge leg up when building bridges or setting ropes for others to follow.
For sabotage they're even more valuable, able to bypass all kinds of passive defenses (walls, barbed wire, even mines) that enemies might leave around their fortifications. Their unmatched mobility lets them make quick, surgical cuts against targets like rail lines, artillery emplacements, or communication towers that other troops can't touch.