The role of aircraft in WWI was threefold, or perhaps fourfold:
- Reconnaissance over enemy terrain. Observation balloons can only be used over the own lines.
- Ground attack, both in a strategic role and in close air support. Strategic bombing did not amount to much.
- Fighting enemy aircraft who do those things, and fighting enemy fighters to protect your own aircraft.
The lack of recon and CAS will make things more difficult for both sides. Getting the breakthrough will be even harder, but there was little enough of that in WWI anyway. The mobile warfare at the beginning was when aircraft were in their infancy, anyway.
Without planes, there would be a much greater role for airships. They might be more survivable in the strategic recon and bombing role without aircraft to worry about.
By WWII, things look completely different, and that part of the question is too wide ... no Pearl Harbour, no Blitz, no Stuka to cover Panzers, no Armored Column Cover.