It's all about space.
Space is insulation- we move apart from others for that buffer.
It serves to keep stars from bumping into one another.
It serves to protect our existence from bumping into others- separates this world from the next, if you will.
The problem with space is, it wants to be filled.
Admittedly, this isn't always a bad thing- the air that fills the space between your skin and your clothing is warmed by your body. The insulation protects you.
When the space between realities fills up, we've got problems.
That's when people like me have to protect you.