So how do Space Marines actually matter in the 40k Universe?
Consider the scale. The population is untold billions spread across an enormous swath of space. There's only around a million marines who are really just kind of off doing whatever they want to.
How are they ever at the right place to be an effective force? It's like having a single SWAT unit for all of the US who is on a continual roadtrip across asia.
I know, I know, trying to make sense of 40K, but this is just the thing that's always bothered me most about the lore. There should be a lot more of them, they should actually be coordinated, or they should almost always show up too late to do shit.
The population doesn't matter. There's only around a million star systems controlled by the Imperium and of that million, I'd guess that a solid 10-15% of them have little to no value and no one outside the immediate region would care if they were lost. Of the remaining 85%, the PDF and Imperial Guard can handle the vast majority of conflicts. Where it matters is when you have a major conflict on a planet that actually matters, which is only a fraction of conflicts.
Furthermore, many conflicts can be won or at least shifted towards victory on the back of orbital superiority. If a planet has no surface to orbit weaponry, the Imperial Navy and Guard can take their sweet time. If there is surface to orbit weaponry, than the destruction of those weapons will be the key to claiming overall victory. That's where space marines come in. They're the ones who drop pod down onto the surface to orbit weapons and take them out of commission. Afterwards, they may or may not stick around but the majority of the work will be done by the Guard, who can land and operate relatively unopposed.
Half of the stories involve the Planet of Someplace getting invaded, sending out a distress beacon, and hey a random SM fleet happens to respond.
[spoilers] still waiting for one where a CSM fleet is the one to save them [/spoilers]
Their significance is not only in the campaigns they wage, but also in their reputation, legends and general knowledge humans have that there are angels of death up somewhere who will come and wreck shit in the hour of direst need (or if you fuck up).
Indeed, countless multi-decades campaigns ran its course without any Astartes presence and most of humans will never see astartes/inquisitor thoghought their lifetimes.
They exist to stop the Lovcraftian horrors humans shit themselves just looking at. Daemon Princes and Chaos Marines are rare, so are Tyranid Hive Fleets and Ork invasions, at least compared to rebellions and bandit raids. The Imperial Guard keeps order and, at a push, fights xenos and rival human states. The Space Marines kill the Daemon Princes, blunt the Ork invasions, execute the Traitor Marines, and scatter the Tyranid Hive Fleets. They are the epitome of special forces, the men who make the regular campaigning possible.
Read the black dragons book. A planet gets helped by marines in gold but turns out they are chaos space marines. They get horded onto a ship to be sacrificed.
The black dragons who look like chaos with horns are actually the good guys but the general populace doesnt know that