In the 2011 film Green Lantern, we see Abin Sur in a space ship going to Earth.
But if Green Lanterns can fly through space, why would he need a ship?
In the comics, this is explained, but the movie didn't explain this at all. Abin was told a prophecy that his ring would fail him when he needed it most, and this would result in his death. As a result, he no longer trusted the ring for space travel and instead used a ship.
See Abin's Wikipedia Page for further details on the nature of the prophecy and how it ties into the larger Green Lantern mythos.
It could be a simple explanation. Just because I can run doesn't mean I want to run everywhere I go. I'd rather drive or ride especially if it's a long trip. Plus you never know what could happen so taking a spaceship was probably a wise choice overall. Maybe he was on a long mission that required him to have certain items, tools, etc. that he couldn't just carry.
It's a good thing he had the spaceship actually because after taking the shot to the chest, that escape pod was looking really nice!
Rings don't have unlimited energy. They do discharge. So, spaceship can either help saving some charge for upcoming fight or they can help survive in case ring go out of charge in the space.
Other than this, it'd be awkward to carry all your stuff (like Lantern or books) with ring field and drop them everytime you land on a planet to recharge.