Because if, asphorylation let's say, turns on a proteins activity, we have to be able to basically turn off the activity of that enzyme by for instance, removing that phosphoryl group. But as I mentioned just a moment ago, this process as shown here, because this is so much more thermodynamically stable than the reactants, this process is essentially a one way reaction. So the question is how can we actually reverse this process? Well, instead of using the protein kinase to actually reverse the reaction, we use a different enzyme known as protein phosphatases. So inside our body it's these protein phosphate phosphatases which are actually used to reverse the effects of the protein kinases. So protein kinases are responsible for phosphorylating that enzyme or the protein.