In short, the defender's advantage is usually one of position or the ability to reinforce more quickly than the attacker.
- Vision / Sight
- Choke points near your base, either existing or made with buildings
- Combat will be close to your unit producing structures
- Structures are well protected behind a choke
It means that if your opponent builds mostly army with his resources, and you build your army and go attack him, by the time your forces get there, he'll have more stuff than you. Thus, your only chance against his army happens if he decides to expand or tech rather than build attacking units.
And this, of course, is where scouting comes in...