It depends on who is using it, Peach actually has much less use for hers, because her float is many times better for any height aerials, and she actually can't do extremely low height aerials (she dips a lot in her 2nd jump, she lands if you do it to early.)
Now I don't exactly know where Yoshi or Lucas impliments them compared to Ness, but Ness has lots of options with his. DJC bair hits nearly below the stage, easily poking through ans shields that have been up for more than a second. Dair, being sped up, can be DJC'd after a dash for a deadly tech chase. DJC nair is one of the best gtfo moves Ness has, and finally Uair can string onto itself two or three times, or get sudden kills at higher %s. In addition, because he can use all these OOS and gets a fair amount of horizontal movement when doing a DJC, hitting Ness shield even from the back is dangerous.
applications I know to work would be (on spacies/Falcon) u-throw->DJC uair->u-throw->DJC uair 2/3 times->dair/bair to finish. This is pretty much guaranteed at low %s, and is only broken of they DI off the stage, or if they DI onto a platform after a DJC uair (if they DI the throw, a full hop DJC will catch them and place you on the plat as well.)
There are more uses, these are just a few lol.