For alpine climbing, it’s often handy to have one or two double length / 120 cm tied runners. Why are these helpful?
Inexpensive. 9/16” tubular webbing is about $0.30 per foot from the bulk spool at the climb shop. You need 9 feet for a tied double runner, so $2.70. A sewn double length / 120 cm Dyneema runner costs about $15. You can have five tied runners like this for the cost of one Dyneema runner. (Frugal climbers, unite!)
Easy to cut and leave behind for rappel anchors. Always get double duty out of your gear - don’t bring extra “just in case” webbing for raps. And, as mentioned, it's inexpensive. You're not going to think twice about leaving behind $3 worth of webbing at a rap anchor, But you may not be too thrilled about cutting up your $25 cordelette. (And you DO have a knife, right?)
With a little work, you can untie it, thread it around a large tree / chockstone / icicle, and retie it. You can’t do this with a sewn sling. (And, it's difficult to tie a secure knot in Dyneema; that's why you can't buy it in bulk like you can tubular nylon webbing.) Yes, untying a knot in webbing after it's been loaded can be difficult. (Needle nose pliers on a multitool are your best friend.) If it's really welded shut, don’t fight it; just cut out the knot and use the rest of the webbing.
Or, you can rig it “bunny ears” style, with an overhand knot loop in both ends; see photo below. (If you rig it this way, buy 6 feet of webbing rather than 9, because you get about the same usable length with less material. Use 18 inches of each end to tie the overhand knots. Adjust shorter as needed by cutting off 1 inch at a time from one end and retying one overhand loop. It should be about 4 feet end to end when you’re done.)
Pro tip - Whenever you're buying bulk webbing, ask the shop to cut the webbing on their hot knife at a 45° angle. This makes it a bit easier to thread when tying a water knot or double fishermans.
If everyone in the party carries one tied double runner, you’ll be equipped to make safe raps off most any route.
About 6 feet of webbing tied bunny ears style. This lets you easily pass the whole loop around large trees or boulders, and also use it as a standard sling if you want to. Read more about the bunny ears cordelette here.