I believe it's to help you orient yourself when you're typing/dialing numbers without looking at the keyboard, much like the bump on the 5 key and the F key on your computer keyboard.
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OK - thanks.
Kinda what I thought - but, even after 25+ years of "keyboarding", I still tend to use the "magellan method" for typing - never became much of a touch typer.
Magellan Method == find a key and land on it.