Why are ancient circles or monument allways surrounded by a ditch?

Question

why are all ancient stone circles and burial sites ie stone henge and avebury allways surrounded by a ditch

Answer 1

To protect the monuments they will fill it with water and grow crocodile in it.

Answer 2

Stonehenge (the stone monument you see today) was constructed inside an existing bank and ditch. It’s some 500 years or so later.

Answer 3

and drainage is a very useful side effect

Answer 4

They are not all surrounded by a ditch but many are. It would usually have been part of the fortifications. Also, the material from the ditch would have been used to build the earthen wall around the site.