In the movie The Martian (2015), Matt Damon is asked to cut a hole in the roof of his rover and cover it with a tarp. I couldn't see any reason why this was done, except possibly to allow it to fill up with more oxygen to use for breathing.
Was this discussed in the book, or was there some part of the movie I didn't pay close enough attention to, where it was explained?
Because they skipped details from the book.
In the book he has two duplicate rovers. He uses the second one for spare parts and as a trailer that is towed by the first rover (in the movie he just uses a flatbed trailer). The second rover has to carry life support equipment that doesn't fit, so he has to cut it open. - reddit.com
I have not read the book but all sources saying the same. From Similar question of Scifi.SE :
From Chapter 20 of the book:
LOG ENTRY: SOL 376
I’m finally done with the rover modifications!
The tricky part was figuring out how to maintain life support. Everything else was just work. A lot of work.
I haven’t been good at keeping the log up to date, so here’s a recap:
First I had to finish drilling holes with the Pathfinder-murderin’ drill. Then I chiseled out a billion little chunks between the holes. Okay, it was 759 but it felt like a billion.
Then I had one big hole in the trailer. I filed down the edges to keep them from being too sharp.
Remember the pop-tents? I cut the bottom out of one and the remaining canvas was the right size and shape. I used seal-strips to attach it to the inside of the trailer. After pressurizing and sealing up leaks as I found them, I had a nice big balloon bulging out of the trailer. The pressurized area is easily big enough to fit the oxygenator and atmospheric regulator.
So its partially adapted scene from book, which doesn't make much sense.
According to the book, the pod was too heavy to make it into space and the dome cover was one of the heaviest solid pieces on the pod. So he had to remove it to achieve escape velocity.
