While this is technically about pre-production concepts for the first Star Wars (1977) film, an explanation is provided in this article on the official Star Wars website for one of the most well known images created by Star Wars conceptual artist/designer Ralph McQuarrie in February 1975; see picture below.
Basically, the initial concept for Darth Vader’s masked persona was based on the idea that Vader—in early drafts of the script—would be “…jumping from ship to ship through the vacuum of space.” Thus this is where the idea of Vader’s suit being able to withstand the vacuum of space came from to begin with.
And FWIW, it seems like this concept painting finally came to life in some way in 2016 considering…
…Darth Vader massacres Rebel troops in the airlock of a Rebel ship at the end of Rogue One (2016). While that airlock may—or may not—still be pressurized, the similarities between that scene in 2016 and Ralph McQuarrie’s illustration in 1975 are too strong to dismiss.
“Perhaps the most popular of Ralph’s paintings to appear in the portfolio, the laser duel, as he referred to it, was completed in February of 1975. While the portfolio, and nearly everyone who has discussed this painting in the past 35 years, describes it as ‘Luke versus Vader,’ at the time it was painted, the protagonist would have been Deak Starkiller from the second draft screenplay Ralph was working from.
It was because of this scene that Darth Vader came to have the look that he does. Ralph, concerned that Vader was jumping from ship to ship through the vacuum of space, felt that he would require some sort of breathing apparatus. George agreed, and the look of the masked villain was born.”