Why are there nearly identical fonts?

I learned from the movie Helvetica that the reason Arial is nearly identical to Helvetica is that Microsoft didn’t want to pay license fees to distribute the Helvetica font so they hired Monotype to modify Linotype’s Helvetica slightly. They just made sure to keep the same font metrics so that a document written in Helvetica would have the same layout and pagination in Arial and vice versa.

But I don’t always understand why there are other fonts that are nearly identical but with different font metrics and/or line spacing; for instance, why are Monotype Corsiva and Apple Chancery so similar? Is it because Apple wanted their own copyright on a font similar to Monotype Corsiva? And why are Bordeaux Roman Bold LET and Monotype Onyx so similar? Is it because Microsoft commissioned Monotype to create Onyx in 1992 after LET created Bordeaux Roman Bold in 1990? Or is there just a “me too” factor involved, in which each foundry wants a product to fulfill similar demands?

I’ve spent some time looking at the differences and similarities, and I’ve noticed that Bordeaux Roman Bold has ligatures and a more extended character repertoire than Onyx, though Onyx is a bit bolder and easier on the eyes. Also, I like the tighter line spacing of Onyx. So it’s a hard to choose a favorite between Bordeaux Roman Bold and Onyx. It’s easy to pick Apple Chancery over Monotype Corsiva because Apple Chancery has a beautiful set of of both common and rare ligatures, more calligraphic letters (especially the slashed dot on the lowercase i), and a much larger character repertoire than Corsiva. When it comes to extended characters, Times New Roman beats Times, but I’ll choose Times almost every time because of its pretty ligatures. If I needed to format a text with rarer characters, though, I would choose Times New Roman. As with the choice between any two similar fonts, it comes down to the application— how many extended characters do you need for what you want to write?

And speaking of fonts whose names have “new” (or “neue”) in them, I’ve found that a “new” version of a font doesn’t always have more characters than the original. While Times New Roman and Courier New have more characters than Times and Courier, Helvetica Neue has a much smaller repertoire of characters than Helvetica. But then Helvetica Neue has all those lovely weights and widths, so it all depends…

Do you know more about the history behind fonts that are nearly identical? If so, please leave a comment and share the knowledge. Thanks!