Beams v Missiles

[Suicide Junkie]

Early on, there is no PD, shields, armor or big fleets.
The range advantage of the missiles trumps regular guns, letting you shoot without taking return fire.

Defenses of any sort quickly degrade the usefulness of missiles towards zero, since as mentioned before, they're just too big to overwhelm PD, but also their damage/kt/turn sucks, and they do not divide among the enemy targets well.