Swarm Missiles can fairly be described as the generic missile launcher of the GearHead universe. You'll probably encounter them in most battles. Like most missile launchers, most of their mass is the ammunition, so they make a good choice for adding just that little extra punch to a mech.
As the name implies, Swarm Missiles have the SWARM attribute, which means that they will divide themselves among potential targets in a given area. For example, if you aim a 5-missile salvo at one mech, with two other mechs within (roughly) five tiles or so, the salvo will divide itself between the mechs and attack all three. Figure two missiles each to two mechs and one lucky mech must only deal with one.
Anecdotal reports on the Gearhead forums note that Swarm Missiles are the easiest weapons to defend against in the game; conversely, they are inexpensive both to purchase and to reload and can be mounted in quantity. In late game, you might consider firing off the whole rack (BV setting: All) at one target. It's difficult to dodge all 20, 40, or even 100 missiles, and being hit even by half will still put a severe dent in most mechs' armor.