Chemical in (s) mixed with chemical in (aq) vs. chemical in (aq) mixed with chemical in (aq)
Short answer is yes. A lot of times you are trying to control the exothermicity of a reaction by doing a slow addition. This is kind of hard with a powder, and if you dissolve it in a lot of solvent, you can add it slower than you would be able to with a powder.

Without the slow addition, the temperature may increase and decrease your yield or selectivity.