There's two smart reasons to mark the middle of your rope. The first one might be rather obvious, the second one perhaps less so, but it may be more important. Let's have a look at both.
1 - Setting up a rappel
Knowing with certainty that the middle of your rope is at the rap anchor is a Good Thing. Of course, if you cut off the end of your rope for any reason, this middle mark becomes less accurate. But most people don’t do that terribly often, so a permanent mark on your rope should last you for a long time. (Yes, you can do the trick of measuring hand spans to find the middle of your rope, which actually is pretty darn accurate, but having a middle mark makes it idiot-proof.)
2 - Belay Safety
This is one people may not think about right away, but it’s arguably more important. For decades, pretty much everyone climbed with a 50 meter rope, and you would never find a single pitch sport climbing anchor more than 25 meters off the ground. But now, many routes are longer, requiring a 60 or even a 70 meter rope. Because these long routes are generally newer, they may not appear in a guidebook, and there's often not a reliable way to tell at the base of the route precisely how long it is.
If you’re belaying the leader, and you notice the middle mark of the rope pass through your belay device before the leader has reached the anchor, that should ring a LOUD alarm bell in your head! You're not going to have enough rope to lower them off and reach the ground, and you need to figure out a crafty solution to a potentially serious problem.
A quick review of the report/book “Accidents in North American Mountaineering”, published annually by the American Alpine club, can confirm that this is a recurring problem. One likely cause is the increase in the number of routes that require a 70 meter rope, as mentioned previously.
Another contributing factor is probably more gym climbers venturing outside. In a gym, routes are almost always less than 30 meters tall, and therefore the 60 meter rope that many gym climbers have for leading is guaranteed to be long enough.
While many people climbing single pitch routes from the ground don’t bother tying into the end of the rope or even having a stopper knot, doing either of these simple fixes eliminates the problem of potentially dropping your leader when you’re lowering them off.
(Even if you have a 70 meter rope for a route designed for it, if those anchors are close to 35 meters off the ground, and you the belayer decide to back up a little bit, that could still cause you to end up short when you’re lowering. So, just having a 70 meter rope does not necessarily eliminate the problem.)
So, how best to mark the middle of your rope?
There’s a ton of information about this on the interweb, have a Google for it and make your own call. Sharpie pens if you’re feeling lucky, designated rope marking pens if you want to be a little more cautious, and sewing a few bits of dental floss through the rope sheath if you want to avoid pens altogether. Or maybe both! I’m a fan of the dental floss, because you can actually feel it with your hand when you’re belaying. If you climb a lot it might wear out or get worn down, but replacing it can be done in just a minute or so.
So, let’s sum this up.
If you’re leading a lot of sport routes outside, you probably want a 70 meter rope, which is pretty much the new standard.
Put a middle mark on your rope, and use it to set up rappels and also for belay safety.
When belaying from the ground, best practice is for the belayer to be either tied into the end of the rope, or have a solid stopper knot tied into the end, to avoid any chance of dropping the leader when lowering.
Think you’d never make a mistake like this? Well, if it can happen to Alex Honnold, it can darn sure happen to you.
In 2016, Alex was dropped by his belayer because they were using a 60 meter rope on a 70 meter route, there was no knot in the end of the rope, and his belayer was not tied to the end of the rope. While she was lowering Alex, the end of the rope zinged through her Grigri and Alex fell onto some “gnarly rocks”.
Would a middle mark have prevented this accident? Hard to say. But it would not have hurt anything.
Read the complete accident account here, from “Accidents in North American Mountaineering”, an annual publication of the American Alpine Club.
Below is a copy paste.
I had run up the route Godzilla (5.9) to put up a top-rope for my girlfriend and her family. At the last second her parents asked us to hang their rope instead of ours. I didn't think about it, but their rope was a 60m and mine was a 70m. I was climbing in approach shoes and everyone was chatting at the base—super casual, very relaxed. As I was lowering, we ran out of rope a few meters above the ground and my belayer accidentally let the end of the rope run through her brake hand and belay device. I dropped a few meters onto pretty gnarly rocks, landing on my butt and side and injuring my back a bit (compression fracture of two vertebrae).
Lots of things should have been done better—we should have thought about how long the rope was, we should have been paying more attention, we should have had a knot in the end of the rope. I wasn't wearing a helmet and was lucky to not injure my head—had I landed on my head, it probably would have been disastrous. My belayer had been climbing less than a year. Basically, things were all just a bit too lax. (Source: Alex Honnold.)