They're normally carrying packets of sperm to get themselves pregnant with when you get them. I bought one for my son's Edge, and it's had quite a few babies I could share out amongst other tanksi don't much like the idea of buying assassin snails to kill other snails and then have them breeding themselves (even if they are better looking).
Do assassins self breed (i.e. if I get one, will it still have kids)?
Apart from the fact that they are better looking, there's also an actual demand for assassins, so you can easily get rid of them, if you do end up with too many.