Do we really need ligatures? As I understand, they originated in the days when it was difficult to mechanically put an "f"-block and an "i"-block close together. Do we have to emulate this in environments where we don't have these constraints?
Even with digital fonts, there are situations where certain character combinations, such as “ffi” touch/overlap each other. Ligatures are a special-case letterform for these situations that blends them more harmonically
Nobody has to, but there's no harm in a little skeumorphism here and there for aesthetic reasons. We have a big walk-in fireproof safe in our office that long ago had the locking mechanisms removed, but the dial and lever are still in place on the door because it looks cool.
I've gotten by fine without them, but designers and typography experts say they are easier to read, and a must-have for serious typography. So yes, we need them.