Why erosion and deposition occur side by side?

It is because that is how it works. If you have a heavy rainstorm that starts to drop large amounts of rain on a field — you will likely see the results of erosion that occurs as the water moves soil from hilly areas to lower areas of the field. Any areas in the field that are flat, all of that water will just turn it into mud. The soil moving from that field is not traveling great distances away from where it was originally located. So, where the erosion occurred may be literally just a few meters from the spot where it was deposited.

in many cases, you will find that the spot of soil deposition will not be very far from the location where the erosion occurred.

Explanation:

answer at the divergent plate boundary in the middle of the ocean;