Note. Unity 5 introduced the Standard Shader which replaces these shaders.
The Self-Illuminated shaders will emit light only onto themselves based on an attached alpha channel. They do not require any Lights to shine on them to emit this light. Any vertex lights or pixel lights will simply add more light on top of the self-illumination.
This is mostly used for light emitting objects. For example, parts of the wall texture could be self-illuminated to simulate lights or displays. It can also be useful to light power-up objects that should always have consistent lighting throughout the game, regardless of the lights shining on it.