I used to send a lot of SMS verification codes. We considered setting up a SIM box, but never did. You get different SMS routing from a phone on a major network than you do from the SMS aggregators, and that could be useful for getting codes to difficult destinations.

But we had enough volume that we could typically get improvements on routing by asking aggregators about difficult destinations (unless the difficulty was coming intentionally from the destination carrier). The aggregators do sometimes use grey routes from SIM farms. Squishyness around terms of use and accounting would have been an issue too, we would not have been able to fly under the radar on 'unlimited messaging'

Another potential use could be if you needed to send a lot of alerts to your employees/customers in a short period. Most aggregators have rate limits, and so do carriers... if you're a big customer, you can probably get limits raised; if you only have an occasional need, you might prefer to have a large number of low cost SIMs.