Slides were initially made with out any weights.
they still worked, untill some one made it heavier by adding soldering wire. this added weight did assist with the speed of getting the bait to a certain point.
now days you have the option of weighted or not.
it's is a personal choice. a non weighted slide may take longer getting to the end stop, so it could assist you having your bait longer in a specific area eg; just behind the lip or back wash of a wave.
just my 2 cents
ss