@fickfickfire: The same reason Goku needs to use Kame to match Vegeta's Galic Gun. To build enough ki to match Vegeta's ki.
Ki blast amplifies their battle power as shown when Piccolo was charging up Special Beam Canon through Raditz scouter. His battle power rises.
Raditz can completely no sell Piccolo's basic moon busting level ki blast with his bare physicals and can even block Goku's Kame with his bare hands but has to avoid the Special Beam Canon because it has a power level enough to cause lethal damage onto Raditz.
Ki is a natural energy that all characters in Dragon Ball have by default. Even Master Roshi's turtle or random farmer with a shot gun.
And keep in mind Vegeta didn't know how turn down his battle power when he first arrived on Earth. That wasn't something he learned until Namek. Before that, the only time his battle power alternated was through his ki blast or Ozaru transformation. Frieza didn't know how to do this either other then his transformations.