Pressed Powder is the travel version of Loose Powder.
You keep the loose powder on your makeup counter at home, and give a dusting after putting on all your face makeup.
You keep a pressed powder in your carrying item, so that should you get oily again during the day, you have something that can take care of it. Because it not very wise to take a loose powder pot with you now is it? Hahaha...
Of course, you can always use blot papers, which i prefer.