For stream fishing weight is added to the fly or leader/tippet, in an attempt to dredge the bottom, just skimming across the bottom. Or, to put the fly down quickly and raise it ala Leisenring Lift to induce a take. But with stillwaters, once in the shoals or near the edge/drop, all depths of that zone are fishable depending upon hatches, your search presentations/retrieves and the vertical route most insects take toward the hatch (excluding crawling across the bottom to shore; crawling up debris/vegetation to surface).
This is a slower probing of the vertical column until, hopefully, a pattern results from hits and/or hookups and your observations of when or where you receive the hits. The tip is to stay in the zone as long as possible. An overly weighted fly will pass through the zone too quickly. A too heavy fly line will pull the fly (weighted or unweighted) through the zone. This is why an Intermediate or Type II line will keep the unweighted fly in the zone longer. With the exception of flies I want to keep at the bottom like a Kaufmann Dragon or the bead head Woolly Bugger, I most often use the clear line to slowly probe the horizontal plain until I lower into or retrieve up into the strike zone. Woolly Buggers can and should also be tied in the unweighted style. Bead headed WB’s offer the up and down undulating, wavy Leech movement. But, sometimes valuable time is lost by passing downward too quickly and staying below the zone. A clear Intermediate/Type I, a Type III and a WF Floating line are the primary lines. Sink Tips and heavier lines could be carried, but 3-4 spare spools plus the costs of the additional lines do add up.