You wouldn't want to use one to climb.
My favourite quote I found on the Internet :
Here's my advice on climbing with grappling hooks: don't climb with grappling hooks.
Real climbers never use them, and for good reason.
You have no idea what they hook onto, so you are trusting your life with something completely unknown.
Those things are just for the movies.
But they can be a good tool for pulling things, grabbing branches, shifting logs etc.
outdoors.stackexchange.com/users/66/Rory Alsop
Grappling hooks are sometimes used in the arborist (tree climbing) world.
Rather than being thrown upwards, they are attached to a line in order to retrieve the other end of the line.
Here is a picture of the relevant maneuver.
The hooks on the end are often curved and rotated so that they will not catch on branches (or castle walls, like a movie grapnel), but will instead slip through the tree and entangle a vertical rope.
I'd say google 'grapnel traversal' , but the search is polluted by a bunch of video games that use the term.
If you are very interested, here is some academic work on the subject.
( Note : it opens a .PDF)
outdoors.stackexchange.com/users/6282/J Kimball
I'm pretty sure grappling hooks were originally used in naval combat to allow for boarding other ships, but some googling found that some combat engineers use them to clear trip wires.
Other than that I think they're mostly just useful for pulling on things you don't mind stabbing.
outdoors.stackexchange.com/users/8226/user8226
In climbing, there isn't a good use.
However, in canyoneering a variant of the grappling hook is occasionally used to escape from potholes.
It is called an octopus.
You make one by attaching several aid climbing hooks like the BD talon to a potshot (a little cloth bag usually filled with sand).
See this book for a picture.
I have personally used them
and I can attest to the terror associated with pulling on an unknown hook placement on soft sandstone.
Thankfully you are usually over water...
outdoors.stackexchange.com/users/5485/Joe Manlove
Living on the Florida coastline all my life and being an avid fisherman, I can testify that with well sharpened points, they were extremely handy for pulling up LARGE fish onto a pier or bridge.
We called them "flying gaffs" and only used them on fish destined for the table.
Places like the Skyway bridge and the piers at Ft Desoto often yielded grouper and kingfish far in excess of the # test of our line.
outdoors.stackexchange.com/users/8980/Garry
Not exactly your movie-style 3-prong grappling hook, but Fire and Rescue personnel often have hook based systems for use as personal escape devices: One such product is the Petzl EXO which consists of a hook, rope, and rappel device.
Rather than being thrown and hoping to catch an edge to ascend a building, they are attached to a building (wrapped around something big, jammed tight against a windowsill, etc.) and used to descend out of a building.
Additionally, climbers do actually use something called a grappling hook , but similar to the rescue products, they are not thrown, but are rather carefully placed by hand.
Once stuck into a hole/crack/flake on the rock, they can be used to pull the body higher up the wall.
outdoors.stackexchange.com/users/13093/orh
Pulling down dangerous tree limbs.
We have had a few elm trees fall victim to Dutch Elm Disease, and one of them has large cracks all over it, and we're afraid large sections of the tree will fall onto people, so we use grappling hooks to pull down the larger, more dangerous branches instead of having to stand under it with a chainsaw.
outdoors.stackexchange.com/users/14804/Daniel Cox
A friend was asking about grappling hooks the other day, apparently they'd seen a mountaineering program where the guy carried a grappling hook. We couldn't quite figure out why, I've never seen anyone use one (outside of a comic book).   Are they mountaineering equipment?  When (if ever) would you use one?  If so how?
Is there any practical use for a grappling hook?
outdoors.stackexchange.com
<gear><climbing><mountaineering>
outdoors.stackexchange.com/questions/10023
outdoors.stackexchange.com/users/None/
Climbers do actually use something called a  grappling hook , but similar to the rescue products, they are not thrown, but are rather carefully placed by hand.  However, in canyoneering a variant of the grappling hook is occasionally used to escape from potholes. They were originally used in naval combat to allow for boarding other ships. Now they are used for pulling on tree limbs, landing fish, shifting logs, etc. They can be used in canyoneering but are not ideal for climbing. They can be used for pulling down tree limbs, naval combat, pulling things and fishing.
They are not ideal for climbing, though can be used in canyoneering. They are useful for naval combat, fishing and pulling on things you don't mind stabbing. They can pull down dangerous tree limbs, pulling things.