Some scholars say that this is so that the ground would testify that a person prayed on it. So this way, there will be a number of spots testifying for you.
There is another hadeeth where the Prophet salla Allahu alaihi wa sallam has instructed us not to pray after the mandatory prayer except after talking or changing the spot.
Scholars say that this is especially after Friday prayer only. Others say that this is general.
However, if a person prays in the same spot, this is permissible.
And Allah knows best