system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] Why are space marines so important to the Imperial war effort, when there are millions of guardsmen per one marine?

In-game they're only about as strong as 3 guardsmen, and while they're portrayed as being much stronger in the books, shouldn't that mean they can take on forces of orks, chaos daemons, or tyranids hundreds of thousands of times their size? And don't space marines use much of the same weaponry as guardsmen, such as flamers, meltaguns, and plasma guns? 

Are space marines that important of a fighting force, or are they just living propaganda?
gpt
A little bit propaganda and a lot of special forces. 


A million guardsmen can take and hold a planet, but it is loud, expensive, and slow. 


Slow is the worst part there, especially when dealing with Chaos. Give the agents of the Ruinous Powers enough time and they'll leave out patsies and move their operations elsewhere. By the time the guardsmen clear the first temple, cultists will have moved to the second temple, and by the time the guard has found the second temple the cultists will have gone to the third and begun consecrating the first one again. 


With the Astartes you have no hesitation- the troops that arrive have been training in urban combat since the guardsmen's grandparents were in diapers. Their armor and speed and weapons mean rapid assaults, quick kills, and a surer result, no having to pause and rethink tactics or issue specialized equipment (void suits, gas masks, etc.)


Much like Marines found elsewhere, the Astartes are a precision instrument, to be applied carefully. They establish the landing zone for the Guard to take, they clear out the worst resistance before the Guard can notice it, they bolster the Guard by taking on the opponents that guardsmen hesitate to engage.