Eab is correct, they can indeed be a rare and valuable unit used for lvling up quickly a unit. As stated above they offer 1000xp by themselves and when combined with the same fairies as the unit you are trying to upgrade (silver, gold, plat or black) will give you a massive 8000xp boost.

Depending on how quickly you get them they can be quite rare or is your like me and use them to lvl gold and above units from lvl 54 upwards you can get quite a saved up stack of them. You can use them however you want but as most will tell you try to use them for gold and above units at a high lvl 40 or 50 is a good starting place (me persoanlly use them at lvl 54), they will save you a lot of gold coin as it is harder to lvl at those high lvls with bronze units.

Use wisley and it is highly recommended you use fairies with them to get the most of the xp boost. Happy lvling!