here is a very simple example to keep you from typing extra stuff
say you want to ensure you are in combat for every line in your macro
you would have a sequence like this
/cast [combat] Cobra Shot
/cast [combat] Barbed Shot
/cast [combat] Bestial Wrath
etc…
now instead of writing the word " [combat] " in every single line, you could create a variable called “C”
in the variables tab you would have C=[combat]
then you can save a few keystrokes
/cast ~~c ~~ Cobra Shot
/cast ~~c ~~ Barbed Shot
[Combat] is an overly very simple example
other classes can have a lot more complex conditions (no-channeling, nomod, talent selection…) that would be a pain to repeat on every single line of code.
variables just cleans it up
hope that helps