With a tx capable SDR, you could have a universal garage door opener, pirate fm radio station, or shut down an airport.
With rx only, read hospital pager messages, sniff satellite calls/texts on the iridium network, listen to police radios while doing other crime(Depending if it’s encrypted or not)
You can do a WHOLE lot of felonious shit with a radio. You really shouldn’t though. One of the most based radio bros, figured out that he could hack the SF bay tsunami siren system with an SDR.