Unless you're playing Araceans in Dark Avatar with Super Abilities turned on (which means you get first strike in combat), your opponent will always get at least one shot in every fight, even if the entire enemy fleet is destroyed in your first salvo. This means you can be attritted down even by ships that are no match for your fleet, a problem if you're operating far from a planet that is building replacements. By having some defense on the ship you can get through many fights unscathed and so improve the time your ships can stay in the field.

I always try to get weapons power close to matching my current victim race's best ships of that size, then pick up as much defense as I can cram in the ship for whatever weapon type they use. Once the initial large fleet battles are done and I'm sieging enemy planets, my ships usually GAIN health after a fight because of extra HP through experience.

Hi!
Check wiki:Ship design strategies.

BR, Iztok