There are two main reasons not to dissolve salts directly in 1 liter of water. At first, the dissolved compounds can increase the volume of the solution compared to pure water. As a result, the final volume of the solution will be higher than 1 liter. Second, if pH is needed to be adjusted, a certain volume of sodium hydroxide or hydrochloric acid must be added to the solution. This will also contribute to the final volume of the solution changing its molarity and buffer strength. As a result, it is required to dissolve components, adjust pH, and add water to 1 liter. In this case, the parameters of the buffer will not be changed.