Suppose you have a u-blox MAX-7Q GPS module connected to a Raspberry Pi, and you need to reprogram the module (for example, to enable/disable certain NMEA strings, change the baud rate, etc.). You could manually construct the various binary UBX strings and send them through
gpsd or straight out the serial port, but that's needlessly complex.
You could also connect the module to a Windows PC and use u-center to reprogram it, but that's a bit of a nuisance too. If the module is conveniently packaged for connection to a Raspberry Pi, then you aren't going to have a readily accessible USB port, nor an RS-232 serial port that you could connect directly to a PC. Sure, you could cobble together a USB-serial interface (or a real hardware serial port, rare as they are nowadays) and an RS-232–3.3 volt level shifter like the MAX3232CPE, or just use this convenient cable from Adafruit, which provides a +5 volt supply and 3.3 volt serial interface from USB, but it's still not quite plug-and-play.
But, there's an easier way that avoids all of those hassles (although it still requires you to have a Windows PC)—enter socat!
$ sudo socat tcp-l:2000,reuseaddr,fork file:/dev/ttyAMA0,nonblock,waitlock=/var/run/ttyAMA0.lock,b9600,iexten=0,raw
This exposes the Raspberry Pi's serial port at TCP port 2000, so you can connect to it over the network from a Windows PC running u-center. You might think you'd then have to use extra software on the Windows side to get a TCP socket to appear as a virtual COM port, but u-center has support for network interfaces built in. Just enter the Raspberry Pi's IP address and the port number, and it will happily connect to the module via socat.