The biggest editing bots tend to do template maintenance (so correcting formatting issues and adding dates to templates). Bots simply do tedious work (that would have to be done anyway) faster - so hence a big edit count.
My bot (which notifies English Wikipedia of image deletions on Wikimedia Commons) edits about three times more than I do each month (and I am somewhere around #500 activity wise).
The very top bot ClueBot NG does more than double the edits of anyone else per month; that is an anti-vandalism bot, it catches about 50% of vandalism on Wikipedia each month. So pretty crucial :) Cydebot (#2) does some sort of category maintenance (not sure what, never seen it before). SineBot (#5) goes around and signs peoples comments when they forget (handy beast).
Basically it is just infrastructure; but because of the nature of the Wiki it is more visible than other forms of infrastructure :)
My bot (which notifies English Wikipedia of image deletions on Wikimedia Commons) edits about three times more than I do each month (and I am somewhere around #500 activity wise).
The very top bot ClueBot NG does more than double the edits of anyone else per month; that is an anti-vandalism bot, it catches about 50% of vandalism on Wikipedia each month. So pretty crucial :) Cydebot (#2) does some sort of category maintenance (not sure what, never seen it before). SineBot (#5) goes around and signs peoples comments when they forget (handy beast).
Basically it is just infrastructure; but because of the nature of the Wiki it is more visible than other forms of infrastructure :)