The most common ligatures involve the lowercase f because of its overhanging shape. Other ligatures also exist—some practical, some decorative, some ridiculous.
Digital fonts don’t have physical collisions, of course. But certain letter combinations might still overlap visually. The only time ligatures are mandatory is when you have an actual overlap between the letters f and i. Check this combination in the bold and italic styles too.
The fonts in the first row above have fi combinations that don’t collide. Those fonts will work fine without ligatures. But the fonts in the second row have fi collisions. Turn on ligatures to correct these collisions, as seen in the third row.
Beyond that, ligatures are largely a stylistic choice. To my eye, they can make body text look somewhat quaint or old-fashioned. If you like that look, great. I don’t. So unless characters are actually colliding, I generally keep ligatures turned off.
Is it possible to insert ligatures manually? Yes, but it’s a bad idea. Manually entered ligatures can confuse your spell-checker and hyphenation engine and generally cause more problems than they solve.
Despite the name, ligatures don’t always connect two glyphs—sometimes they create separation, as in the italic
I mentioned ridiculous ligatures—at the top of my list is the
Thligature included among the default ligatures in certain Adobe fonts, like Minion. It’s frippery, amputating two perfectly good letters to make one ungainly hybrid. Worse, because This such a common letter combination, this ligature shows up all the time in body text. Just say no.