Alkalinity question

[spawn]

Being still fairly new to the hobby, why does everyone dose alk? I recently started using two little fishies a & b and it caused an alk spike ( ^-13 ) needless to say I stopped dosing. Prior to using it my alk had never been above 9-10 dkh It's now back to 9. So I'm wondering what causes the drop in alk that people dose to counter-act. My tank is now only 5 months old, it's pretty much fully stocked with sps. Is alk consumed?