They have a private surgeon, and a cook, and a tactical analyst, and a team of armourers who can work like an F1 pit crew when they have to. Every one of them has their own retinue and support staff, who probably don't see the front lines most of the time, and they interact with them a lot. They're elites, and they're treated that way.
Space Marines are often portrayed in a sort of "Band of Brothers" style, with the shared nobility of fighters who risk their lives for one another. They ensure that someone is doing that for them. Not to maintain armour, or lead prayers, or prepare gallons of nutrient gruel. They have a job, and that job is to be ready to do extraordinary violence. There isn't an exact equivalent for what a Marine is in modern culture it's something between a knight and a fighter jet. This is how I tend to think about marines too.
Servants of the Emperor in dire straits are encouraged to look to Saint Ashe's example as what they may strive for in the face of the enemy, and to the grim purpose of cutting the unclean (from the body or the communal whole), whatever pains it brings, rather than allowing unbelief or corruption to fester. But while all others fell to wounds or despair, Ashe struggled on, and when his very flesh was tainted by the Enemy he had the force of will to cut the corruption from himself and replace the unclean hand with a chainsword, which he went on to use in glorious struggle against the Emperor's enemies, and such was his skill and leadership that despite his humble origins he was dubbed the King Uncrowned by his allies.
As the legend goes, he was a humble shopboy whose friends found a work of heretical knowledge, and read it rather than taking it and themselves to the Ecclisarchy, which naturally lead to forces beyond mortal ken coming to corrupt and destroy them.