To start off with, what you are asking is a sort of 'what if' approach to history since we cannot tell for sure what the outcome of WWI had been if aircraft had not been there. Also my answer here will be a bit short since I'm a little cramped for time atm. I will extend and substantiate with sources later.

But in my opinion the impact of aircraft on the overall course and outcome of WWI was quite slim. By the start of WWI the aircraft was pretty much still a novelty form of transport only around for a decade or so, as such it was still a field of experimentation where probably more airmen died in flying accidents than combat.

Quite obviously: The main strategical advantage of aircraft was that they allowed access beyond enemy lines, without having to break through them on the ground. Keeping that in mind initially aircraft were deployed in observatory roles to replace the balloons that had been used in that role in combat since the battle of Fleury in 1794.

This then developed into an arms race of sorts as either side sought to take away the advantage of the other side having observer aircraft. To this end pilots were initially equipped with pistols to fight off enemy aircraft. Not very much later machineguns were mounted to aircraft A big break through was the synchronisation of machineguns with the propeller, allowing for the mounting of machineguns along the longitudinal axis, making aiming and fighting a proper possibility for the first time. This developed into a race to develop ever faster, more maneuverable aircraft and aircombat tactics.

Another, different usage was found in strategic bombing the hinterland of the enemy, basically a new take on the cutting of supply lines. Unfortunately the aircraft (and also Zeppelins and airships!) of the era proved to unreliable and to short ranged to have a true devastating effect on the means of production, although the Germans did manage to drop some 300 tons of bombs on Great Britain causing some 1400 casualties. (to put into perspective, the allies are estimated to have dropped well over 3 milllion tons of bombs during WWII) Eventually these bombing campaigns were abandoned because relative to their cost they yielded to little true results.

The big impact of Aircraft in WWI was not so much in true results, first of all the lessons learned and developments were very important for WWII. Secondly, the exploits of the aces that were created gave the homefront a moral boost, and thirdly the thought that the enemy could hit at home lowered enemy morale (or at least was thought to do so)