This may be over-stating the obvious:

Depending on how long you make the roll top, it can give you the option of an extension sleeve for the times when your load is bulkier than normal. Of course, you can accomplish pretty much the same thing with a couple of lash points and a stuff sack.

Drawstring with lid pocket has the advantage of giving you someplace to store your maps, or first aid kit, or other small items where they’ll be handy as you hike.