Why use borders around dropdowns in a mobile app?

I'm working on a mobile app with a GUI similar to the Gasbuddy mobile app.

The first screenshot is what Gasbuddy looks like now. The 2nd screenshot is the same thing but without the borders around the dropdowns.

I may be overthinking, but is there a UX advantage to adding the borders to the dropdowns?

Also, in mobile apps, are these controls called dropdowns, comboboxes, or is it called something else?

But I've seen apps that use dropdowns similar to the ones below without borders: