The film (and the screenplay) don't elaborate on this, but the book does: Beck had to crawl on the hull because when they blew the tip of Hermes, they blew one of the doors, and you need two doors to be able to enter the ship1.
“We’re going to have to literally blow up one of the doors,” Lewis explained. “I’d rather we kill the inner one. I want the outer door unharmed, so we keep our smooth aerobraking shape.”
“Makes sense,” Beck responded as he floated back into the ship.
“One problem,” Lewis said. “I want the outer door locked in the fully open position with the mechanical stopper in place to keep it from being trashed by the decompress.”
“You have to have someone in the airlock to do that,” Beck said. “And you can’t open the inner door if the outer door is locked open.”
“Right,” Lewis said. “So I need you to come back inside, depressurize the VAL, and lock the outer door open. Then you’ll need to crawl along the hull to get back to Airlock 2.”
“Copy, Commander,” Beck said. “There are latch points all over the hull. I’ll move my tether along, mountain climber style.”
Chapter 26, emphasis mine.
1 Strictly speaking - he could enter the ship, but the lengthy process of repressurising it (they evacuated all the air, after all) would not allow him to make it in time.
As pointed out by @davidbak, Hermes in the film is different from Hermes in the novel.
According to Andy Weir's description in a Reddit AMA, Hermes was different in his view:
I imagined an Hermes as being a very large cone-shaped ship, like a super-sized Orion capsule. It was split down the middle and the two halves could separate, attached with cables, and spin to provide artificial gravity.
It would look like a blend of the two pictures below:
The left option. The heat shield was not split. It remained a solid circle in the center.
E.g. the upper half is a solid cone, and the lower half is two halves of a cone with its top cut off.