Space is big. Really quite absolutely huge. It's so big that one might argue that it has some sort of inferiority complex. It's as if it's just saying, "Hey, look at me, I'm BIG!" Its bigness gives rise to all kinds of problems for us.
One of those problems is that it's big enough to contain other intelligent life forms. We use the term intelligent loosely, for no matter how big the universe is, there are other beings who will want what you have. Unless we plan to hand it over, we will need to defend ourselves. This comes in the form of putting things on them that will cause harm to our enemy's ships.
And so that is where space militarization comes in. Now we can go down the path to researching weapons and defenses for our vessels.