The purpose of a sightscreen is to provide a clear background against which the striker can see the delivery more clearly. Sightscreens are usually white, against which a red ball will stand out. In those limited-overs matches in which a white ball is used, the sightscreens are usually black, for the same reason.
Sometimes the striker will ask for the sightscreen to be moved, especially if the bowler changes his bowling action, from over to round the wicket, say.
As sightscreens must always be outside the boundary, it is good practice to mark the boundary in such a way as to leave room for any subsequent movement of the sightscreens that may be required during the match.