Long distance wireless networks, I guess?

Although I don't think it would be possible in countries where frequencies belong to operators, frequencies is a problem for SDR folks if they want to do long range.

I guess cybersecurity agencies would also not be happy with people using this, since it could be used as some form of stingray, I think?

But in africa or other third world countries, I guess there are large areas without internet, so they could obviously benefit from cheaper ways to have internet over there, if it was built for free or funded by some NGO.

In some countries you can get temporary licenses for running GSM networks during events like concerts, especially in remote locations.

I would envision applications like emergency/disaster management, event management, temporary remote network (e.g. desert, mountains), etc. in situations where regular network is down or very crowded.

You could build a test lab for examining the attack surface exposed by cell phones to their mobile carriers.

In a Faraday cage, of course.