What exactly are you supposed to be doing with them/use them for?

Can't say they're for teleportation points because it said If you go to far from it, it automatically get placed back into your inventory. rendering that useless and the manual doesn't say what they are used for

As the topic says they really don't do anything

The mechanics;
You need two in your party inventory to work them.
Step 1. Place (drop) one on the ground & walk away from it for a while.
Step 2. Open inventory and right click (then select use) the other pyramid.
Results – the entire party will be teleported back to where you placed (dropped) the first pyramid.

Restrictions;
Leaving one map area for another will reset both pyramids to your inventory so for example if you travel from Cyseal (the starting map) to Hiberheim or "end of time" then you are deemed to have left the map and your pyramids will reset.

Strategic uses;
Getting past tricky terrain, for example give one to your ranger whom sneaks invisible past some guards and then have the wizard character activate his pyramid to be teleported next to the ranger… thereby sneaking the whole party past safely. There could also be a trap or pit you need to jump over but only the ranger has a teleport ability so the pyramids are handy for that as well.

Tactical use;
You can use the pyramids to fight a retrograde battle… For example, lets say you want to fight past a large group of guards to enter Hberheim but they are proving to be too difficult at your current level.
1. Back up away from them enough that it takes more than one turn of normal movement to reach the guards (say 5 yards in game space) and drop one pyramid on the ground.
2. Now go and engage the guards, concentrating your firepower to kill some key target like a mage or priest.
3. Once that target is dead you have the character carrying the other pyramid use it (during his turn to act) which teleports the entire group back to where you left the first pyramid… thus giving you time to heal and run away before the remaining targets can catch up.
4. If the enemy guards keep following your party then you can just spend a couple turns leading them away before activating the pyramids again and effectively jumping over their lead melee fighters to reach the softer targets at the back.

Their use is actually explained a lot better here than in the original game, Divine Divinity. ussnorway brings up some interesting tricks that didn't really work in that other game, in part because this is a game with party mechanics.

I'll agree that the general travel usefulness is limited by greater use of portal travel in the game, which you can activate anywhere. But then we go back to those things ussnorway mentions, which really does change how they can help you get around your immediate area, whereas in DD they were mostly useful for utility travel - for instance, returning to a merchant to sell your latest loot grab and then going right back to the dungeon area. In D:OS I did use them a while back to get past that giant acid field in the Cyseal Villa, so that was a big help.

My brother and I play together online. We each keep one in our inventory and can insta-port to each other's location, so they are very useful in multi-player play. One of us will go to town and sell and then port back to the other, or if we just get separated we jump via pyramid to save running across town.

Haven't tried using it in combat yet, but may try giving it to a melee character and have them charge in and do a whirlwind attack, then port out of the enemy mob via pyramid.

Edit: tried it in combat and it cannot be used, so no pyramids allowed during combats.

Everything people are saying should have been explained in examples by the devs. This proves my point about why things in games must be explained.

It only proves you're a unimaginative person.

I was able to get into the castle in Hiberheim with them! =)