An if by itself either runs or gets skipped. But what if you want to do something different when the condition is false? That's what else is for.
int age = 15;
if (age >= 18) {
System.out.println("You can vote");
} else {
System.out.println("Too young to vote");
}
Exactly one of the two blocks runs. If the condition is true, the if block executes and else is skipped. If the condition is false, the if block is skipped and the else block executes. There is no scenario where both run or neither runs.
This gives you a clean binary branch. Every input goes down one path or the other, so your program always has something to do.