I would typically be inclined to go with the majority opinion that the finger is the best safety, or that a holster is ideal, but the device does have a niche:
A holster off body often requires two hands to quickly remove from a gun.
So a gun in a purse for example that is holstered will likely prevent the owner from reaching in and making use of the firearm, especially if the holster prevents cycling.
A device like this however can obviously be removed one handed, and so would not require two hands to allow immediate use of a firearm.
People also pocket carry things like the Kel-tec P3AT, Ruger LCP, and various revolvers, and such a device could add some extra safety from a depressed trigger for a gun carried loosely.
One of the biggest dangers outside of a holster is a foreign object working its way into the trigger guard. A battery, pen/pencil, eraser, key, coin, lighter, knife, or other typical object that gets wedged between the trigger and trigger guard can discharge the firearm without any direct manipulation outside of moving the bag.
Such a device would reduce this danger.
There is of course always the danger that an additional safety mechanism may cause some to become even more careless or casual because of a decreased perception of danger. That is dangerous but is not really a basis on which to judge the product.
That said I do not imagine the device will always "snap" in, and may be looser on some guns or easily fall out, or begin to after some wear to the trigger or device.
That would make it more gimmicky than practical, but a better one that was adjustable or applied spring tension might better compensate for that.
It does seem a little counter intuitive to me to be playing with something that touches the trigger of a loaded firearm when not intending to fire that firearm though, even if it is touching the other side.