[Taqwus]

Fighters can also have higher combat speed than any ship, which is nice for nailing fleets whose ships are designed for long-range fighting.

And, of course, they're nice against missile-heavy fleets...

But yes, the main use would be emergency defense. Fighters don't take maintenance, and ones in planetary storage don't count against the unit limit, so you can stockpile large numbers of fighters. As long as you have early warning (like cloaked sensor/scanner satellites up to two systems away from your colonies), you can launch 'em faster than you can get a defense fleet there. Unless the game's progressed to warp-point-opening combat, of course, in which case offense wins. *shrug*

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-- The thing that goes bump in the night