It's not about relating with super humans, or better humans, it's about being inspired by their values. The values that the Imperium upholds are also values that we in our life can appreciate and agree on. Not talking about religiousness or any crap like that, but perserverance, will, honor, righteousness, etc. Sure, none of us are in outer space sawing poor people in half but we all have on our battles which at times can be hard. At those times we can be inspired by the thought of enormous supersoldiers that could do anything they put their mind to or about mere imperial guardsmen that held the line despite overwhelming odds.

You're absolutely right, it's similar to marvel heroes, but much more focused on values and ideals. Sure supersoldiers are cool but what makes humans in warhammer special are their ethos.

Also, that's not a fair comparison: humans in Warcraft are the single most boring race to play in any rpg. The fact that they are second is already far too much credit.

Also, the thing is that for you to understand a story about the imperium, you need pretty much no prior knowledge of 40k. To properly understand a story about any other race, you're gonna need a lot of backstory.