They don't but they don't reduce it either. They just look pretty. Some people like it while others do not.
The problem with the IT community is we often try to argue the personal preference using some bastardisation of the scientific method. So you'll get people make claims about readability et al. But really it's just personal preference.
Personally I quite like them when used with a typeface that doesn't go nuts with ligatures. Hasklig is a great example; it's based on Source Code Pro and only really uses ligatures for character combinations that are generally only used together in an ASCII art kind of way. But that's just what I find pretty; others will undoubtedly hate it and have their own reasons too.
Sonny advice is just experiment. If.you find it pretty then use them; if you don't then don't use them.
I agree. The ligature thing looks neat at first but I have spent far too many years looking at regular ASCII text.
I mean, spot the difference between ==, = and =. Or <= and ≤. I especially hate != as ≠.
And since at some point we have to use a different editor or read a git diff, we need to read the regular ASCII form anyway. So what's the point and why bother learning ligatures?