Welcome to the wonderful world of macros for those who are lazy or have filled up most of there action bars macros can be very handy!
Say if your a mage and want to do some burst damage you can make a macro to cast Multiple spells (also if you have trinkets with use effects you can add them also to the macro to boost the damage)
Heres some simple examples for a mage.
/cast Presence of Mind
/cast Icy Veins
/use
/cast Frostbolt
It does exactly what it says once you use it it will cast a instant frost bolt which will also benifit from the Icy Veins and the trinket. This macro can then be repetably pressed and will just cast Frostbolt untill the cooldowns have finished.
/cast Presence of Mind
/use
/cast Pyroblast
This casts a instant pyroblast with the trinket activated at the same time.
This can also be applied to elemental shamans
/cast Elemental Mastery
/use
/cast Lava Burst
This works extremly well if cast after using a Flame Shock however when you include Flame Shock this affects the macro cooldowns and the macro will only cast flame shock.
Well that should get you started macros can be applied to pretty much any spell just experiment and find the ones that suit you and that work happy macroing!