Should this also be used to fatten up leads ? make them sound les thin ?
You could try it and see how it works out. But I can't think of a time where I would double track a traditional solo type lead though. Despite the immediate wow factor when double tracking stuff, there are often downsides and that can be punchiness. Like a good, punchy, fat vocal, lead guitar often has the most impact when mono (maybe some stereo effects such as delay though). Double tracking and panning have the effect of pushing it away, creating space and air.
I would be most likely to use it for massive sounding riffs, chords and cleans.