The newly issued shares get sold by the company and the company gets that money for their investment/operations/whatever purposes.

So the new shareholders pay "into the pie" and the pie gets bigger, and the effect is that the old shareholders have a slightly smaller share of a slightly larger pie which, if the new shares were sold exactly at the market price, theoretically should have the exact same value - and in practice, assuming the decision to raise capital and invest is useful, usually results in a larger value.