The most common ligatures involve the lowercase f because of its unusual 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. Ligatures are mandatory only when you have an actual overlap between the letters f and i. Check this combination in the bold and italic styles too.
The font in the first row, Concourse, has an fi combination that doesn’t collide. That font will work fine without ligatures. But Equity, in the second row, has fi (and other) 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 bestow a somewhat quaint or old-fashioned look on body text. If you like that look, great. I don’t. So unless characters are actually colliding, I generally keep ligatures turned off.
text-rendering: optimizeLegibility (or better still, both)
Is it possible to insert ligatures manually? Yes. You can either insert them as you type from a character palette, or you can search and replace at the end. In HTML, you can enter the escape codes for the ligature glyphs. But I don’t recommend this. Manual ligatures can confuse spelling checkers, hyphenation engines, and search indexers, and generally cause more problems than they solve.
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 amputates two perfectly good letters to make one ungainly hybrid. Pure frippery. Worse, because This such a common letter combination, this ligature shows up all the time in body text. Just say no. (To the ligature, but also to Minion—see Minion alternatives.)