If you're planning to dive in the cold, dark, usually poor visibility waters of the North Atlantic then you definitely need a reel. You never tie it to the downline, always to a piece of the wreck close enough to the tie off point so that you can find it. The tie off to the dive boat can break free and just think about what might happen if you are tied to that line. Nothing good will come of it, I'll tell you that much.
That's for starters. Another think you need to know is that if you decide to penetrate deep into a wreck, using a reel as a guideline, you want to tie it off every few feet, otherwise it can break when caught up on a piece of wreckage, or it can sort of disappear into part of the wreck as you go around the corner- and when you return back towards your entry the line seems to go right into a wall or whatever.
Of course there's more to running a reel than just that but those are certainly two of the more important takeaways.
Another that comes to mind is that if you cross another diver's line, you run yours under it, it's a courtesy "right of way" sort of thing.
Yet another is that you need TWO reels and a marker that you can deploy from depth should you not be able to return to the upline for some reason or even if it should break while you're off exploring due to no fault of your own.
And now that you have me thinking, another tip is to always know where your line is relative to your position, and never let it get too slack or it tends to develop a mind of it's own and wrap itself around parts of your gear that are difficult to reach.
I use "The Reeler" by the now defunct Spectrum Manufacturing, which is a completely sealed unit, the string is inside and it's almost impossible to tangle or go off the spool, and the Mantra which has an integrated grip in the handle so to let out line all you need to do is squeeze as you swim along.
