As a developer it is handy to know different commands to do things from command line easily. Graphical user interface is good for most users but it is slow compare to terminal command. If a developer is familiar with command line and knows different useful command then he can do things quicker compare to non-command line developer.
Following are some really useful command line command for developers:
Kill all the running java processes
$ killall -9 java
Ports that are being used
$ sudo lsof -PiTCP -sTCP:LISTEN
Find process locking a port
$ netstat -vanp tcp | grep 3000 or $ sudo lsof -i tcp:3000