The real question is why were Moon missions planned in the first place, because the answer to both questions is the same. The vague consensus among those who've looked into the Apollo hoax missions in great detail is that NASA fully intended to go to the moon, but they discovered about half way (or more) through the decade of build up to first launch that they could not send humans into space and have them come back alive. By that point the propaganda build up to the missions was extreme, as was the spending, construction etc. It was decided to not reveal a weakness and disappoint people and humiliate themselves by revealing cold hard reality, and to use their vast funding to fake the documentation of the missions instead. The many training projects and models built in the decade of run up to the missions contributed to pulling of the fakes.

If you have more questions then you really should watch American Moon, since probably every question you could think of to ask on this topic is covered in that documentary:

https://youtu.be/KpuKu3F0BvY