Those runway-looking things

As far as I ever got in PSIII, I never understood what those things on the overworlds are for and don't recall ever having to use them. Then again I only just now started playing it again last week. The farthest I ever got was Ayn marrying Sari. If you don't know what I'm talking about:There's one near Hazatak for example.

The runways are in every world. They let Wren turn into his aerojet mode (fast travel and skip encounters, hooray!) but he needs the Aero Parts first.

Mieu: (looking at the box of parts) Wow, this looks really complicated. You're sure you can pack all this into your body, Wren?
Wren: I am sure, Mieu. Please turn around while Prince Sean installs the Aero Parts.
Mieu: Wait, what? Why do I have to turn around?
Sean: It's an android thing, Mieu. Just turn around.
Mieu: But I'm a biodroid-
Kara: (turns Mieu around) There you go. Get crackin'.

What Snorb said. Basically, the way it seems to work is that Wren-type droids have three upgrade slots- 3-Wren uses his for his transformation slots (Aero, boat, submarine- good god, he's Rush) 4-Wren uses all of his for guns.

Yes, that's the fact : the overworlds are the ways for Wren to transform !