The reasons for this are unknown. It is speculated that they believe the ellipsis adds an air of mystery or intelligence to their post. This second point can be confusing, as many ellipsis folk are awful spellers, and often unable to make use of even basic logic. It may represent the ellipsist's own speaking style, perhaps reflecting a quiet voice, or a tendency to trail off.
All that can be said for sure, is that it is really fucking annoying.
"WHY CANT YOU SEE WHAT IAM TRYING TO SAY......? I AM MAKING IT AS CLAER AS POSSIBLE AN D STILL YOU DONT UNDERSTAND.....! ITS SOOOOOOOO SIMPLE....YOU MUST BE VERY DUMB.."
"ellipsist pls go"