While there are seals that one could use, they claim that these seals can be easily broken, not to mention if one takes their time, could replace it with a new seal in almost the same same spot, leaving you to think none the wiser. The idea behind nail polish is to create what seems like a random pattern which makes it harder for the tamperer to replicate. The use of glitter nail polish is similar to what astronomers use to detect changes in the night sky, called blink comparison. Because each application of nail polish seemingly creates a random pattern in the glitter, it would be almost impossible for the tamperer to reapply it in exactly the same way.
Of course once the device has been tampered with, and with data possibly stolen or malware installed, it might be too late, but at the very least you know that it has been messed with and you can take the appropriate action, as opposed to finding out at the last minute.