To be clear, the spacesuits in Ridley Scott's Alien are the only thing Moebius designed to make it into the movie. (Although much of the inspiration did come from Heavy Metal magazine, so Moebius probably had more influence that just that.)
Of course, they're cool spacesuits.
At its core, it's a simple horror movie -- a terrible monster is on the loose and kills the crew of a spaceship one by one. There's not much more to it than that. And sure, that might be scary anyway, but in Alien, all the elements of design add up to make it terrifying.
H.R. Giger gets most of the attention here, and probably rightfully so, since he is responsible for most of what viewers remember, but everything from the interiors to the costumes make this world believable and uncomfortably claustrophobic even before the alien shows up.
So while something like the spacesuits may seem like a relatively minor thing in the movie, they do contribute to the overall feel. They are both sinewy and bulky and the bubble helmets with overhanging lights make the characters feel shut in and restrained even when they're out of the ship.
And that feeling of being stuck is what makes the movie work. We've all seen movies where characters who don't know or necessarily like each other are put into a terrible situation and have to learn to work together or trust each other. That's nothing new. And while the script and the acting is impressive, that's not quite what makes it so scary. They are trapped -- that's where the terror comes from.
Alien remains scary because all of the elements add to one mood. Even if one of those elements just seems trivial at first -- they're just spacesuits, for instance -- it would've been a lesser movie without it.