Why we add serial ports
A normal user will interact with the WRT54GL with a pretty web interface, which runs as a stripped down web server that listens to port 80 and provides access to basic configuration opitions.
When we load our own code to be executed, we are starting with a blank slate--anything we want the hardware to do we have to code ourselves. This makes the first steps very difficult: we can't exactly check the web interface to see if we're doing things right.
How can we know that our code is working properly if there is no way of getting feedback?
Enter the serial console. A serial connection is essentially two wires (transmit, and receive) over which two computers can agree on timing and transmit bytes to each other. In our case, these bytes will represent characters, and the characters will be the output and input of a console. LinkSys actually includes the hardware for two serial ports on the WRT54GL as headers on the circuit board. OpenWRT (and LinkSys..?) runs a console on first serial port by default, so that once a serial connection is properly made, all one has to do is press enter for a console to appear.
The serial device is a much more basic service than say, communicating over the network, so it is a likely candidate for our first form of communication to achieve. We could have also tried for maybe the LED lights, but it would be incredibly difficult to intepret output and also impossible to get input using them. So the serial port was the goal.