primarily, paratroopers can be used for four Things:
1) encircling divisions on small fronts, like North Africa, dropping paratroopers behind the enemy can cut off their supply and allow greater chances of decisive victory in that theatre. This strategy has been mostly stopped in mp against compextent players but every once in a while does work.
2) victory point rushing to force rapid surrender. Normally checked in mp games (having at least one division on the capital etc) but the odd rare para drop on Paris every dozen games can work.
3) port/island captures. Not very effective.
4) rapidly expanding a front. effectively landing 1-3 tiles behind a frontline in a concentrated area ( for examples, using 5 20 width paratroopers and dropping them Adjacent to each other)can effectively allow for a large breakthrough to occur once the enemy frontline tile has been broken; as the enemy can no longer retreat or fill the tiles now occupied by the paratroopers it means things like armor pushes in France can become quite rapid and quickly outrun enemy attempts to resolidify and reorganise a line a line.
it seems paratroopers never fair particularly well attacking divisions from the sky, so the best use for them now is either for encirclement or using them as reare line tile grabbers to effectively allow front line breakthroughs to occur More quickly and effectively.