Divers use spools to deploy Surface Markers so that they can deploy deeper while using much lesser gas, and be assured of an extremely inflated Surface Marker due to gas expansion.
Cave Divers use spools to lay jumps, whereas reels are for laying primary lines in an exploration. Technical Divers prefer spools that have a certain density, like the DIR Zone Delrin Spool or our own HD Aluminium Spool. In any event, if the diver lets go of this spool when deploying the SMB, it will sink back to eye level and the diver can grab it.
A full stainless steel kind of spool will sink all the way to the bottom like an anchor, whereas too light of a spool will float away in mid-water in the event that the diver loose hold of it when deploying.