OPTIS (Odyssey Pneumatic Tyre Insert System) let you run lower pressures for better grip, without adding significant weight.

Compared to a normal foam insert, OPTIS can be fitted and removed much the same way as an inner tube. You can tune the feel of your bike by changing the pressure of the OPTIS, making the tyre more progressive, in the same way as you can tune your fork by adding tokens. 

Inflating the OPTIS pushes the tyre onto the bead, so inflating tubeless tyres is easy, and the bead is locked onto the rim, preventing burping.

OPTIS is manufactured from material much tougher than an inner tube, so the OPTIS doesn't inflate to fill the tyre, and it'll take a huge impact to damage and puncture it. If you do manage to puncture an OPTIS, you can patch it with a normal puncture repair kit. Worst case, you can replace the tube without the valve - also helpful if you change wheel sizes.

Another huge advantage of an Optis over a standard insert is what happens when you manage to get a flat in the middle of nowhere. Granted, you can take a normal insert out and stick a tube in, but what do you do with the insert? Using an Optis makes taking the tyre off much easier, and an Optis can be rolled up and carried much easier than most inserts if you have to. 

Each OPTIS weighs about 90g, so it's lighter than normal inserts. The weight is easily offset by allowing you to run lighter tyres or rims.