Why was deionized water boiled before measuring pH

You mean distilled? I know you're a newbie but please be more specific...

No, he means deionized water.

No no I meant distilled instead of boiled, as the procedure is similar!

Mu guess is that it was boiled to remove dissolved oxygen.

 

Cheers,

 

O3

Or simply clean up the salts and all what's in there so you can get only water!

That's what deionization does. The water's already deionized.

It's boiled cause its fun to play with fire.

Usually it's done to remove any dissolved gas (e.g. CO2) that might affect the pH.