There are several different, sometimes overlapping markets for them.

Reptile and amphibian keepers use them, along with live plants, to set up a bioactive tank that is relatively self-cleaning and self-maintaining.

Keepers of very small reptiles sometimes maintain colonies as feeder insects.

And still others skip the scaly middlemen and keep isopods as pets. They can get hilariously expensive. Just a couple of years ago, rubber duckies (featured on this site) used to be $90 a pop due to their rarity and difficulty in keeping them. Since they exploded in popularity, however, they're a much more reasonable^1 $15 a head or so.

[1]: For certain definitions of reasonable.