The use case is fraud. You need thousands of burner SIMs to send millions of spam texts. But the way it works, every SIM needs a radio (the negotiation with the network is very heavyweight compared to an SMS transmission, you wouldn't want to bounce between them by constantly reconnecting). So you need thousands of radios.

I've read that at least for for GSM VoIP gateway setups, people typically rotate SIMs because it looks suspicious for one customer to be making calls all day. In fact there is a whole industry most people have no idea about dedicated to detecting these so-called grey routes. Having perused some of these offerings, it seems fairly typical to offer 16-32 radios and 256 SIM card slots. For residential proxy setups, I've seen a bunch of 16 port USB hubs with Huawei LTE modems.