The Apollo service module carried a lot of propellant for performing the following tasks on a Moon mission:
- Mid-course correction manoeuvres to and from the Moon.
- Braking the entire CSM/LM stack into orbit around the Moon.
- Escaping low Moon orbit to transfer the crew back to Earth.
Especially the two later are relatively expensive manoeuvres, and the same propellant in a CSM configuration without the LM sums up to multiple $km/s$ of $\Delta v$.
What was all that propellant used for when the Apollo spacecraft did not travel to the Moon? Examples are the early hardware tests in LEO before the Moon missions, the three missions to Skylab and finally the Apollo-Soyuz test project.
Was the SM close to empty? Was it used as a last stage to reach orbit? Did the Apollo spacecraft perform some expensive plane change manoeuvre? Was a lot of fuel launched just to be ditched at re-entry?
None of those possibilities makes very much sense.