First off, thank you Obsidian for even allowing the option to enable/disable ligatures in game (they are found in Graphic options, one of the check boxes). It shows a commitment to detailing, even if it would probably be considered of negligible importance to most...
I would hazard a guess that most aren't familiar with ligatures--it's a detailing thing in the world of graphic design and typography. Here is a very succinct summary. They basically improve readability, so certain combinations of letters don't look smushed or interfere with each other--these are typically problems with 'fl,'