I just bought a wired mac keyboard. Where I would expect numlock to be, it has a key with this mysterious symbol on:
⌧ (a rectangular box with a cross through it)
What is this button?
This is Clear key.
A Clear key, instead of a Num Lock key, on models with full numeric keypads, as these are dedicated to numeric input and not generally used for cursor control. In Unicode, the Clear key is represented by U+2327 ⌧ x in a rectangle box, defined as "clear key"
In general, its function is to clear the selection, much like pressing Delete or Forward Delete, but unlike with the Delete keys, it will not do anything without something already selected. Luckily, in the Finder, it selects the last item alphabetically, rather than "clearing" selected files and folders, which could be bad.
It functions as a normal Num Lock key when running Windows or Linux on a Mac, either through a VM or natively. This can be frustrating though, because even old Apple USB keyboards that have "Num Lock" printed as "alt" text on the Clear key don't have an indicator light for that key (though older ADB keyboards did).
A long time ago, in a Mac OS far far away, I recall occasionally coming across Mac apps that used the Clear key as a regular Num Lock key, including a very old version of Excel. I don't recall seeing any Mac apps use it in this way in recent history, perhaps not since the release of OS X (or the obsolescence of the ADB keyboards).
The "clear" key is used to clear simultaneously the contents of all selected cells in Microsoft Excel. This allows the user to clear more than one cell at a time, which is what the "delete" key is limited to.
This function of the "clear" key is essential for work with spreadsheets.
It's a clear key. In apps which support it, it clears the contents of the selected entry box. For example, in spreadsheet apps, it will clear the contents of the current cell.
You can possibly use it to wake up your computer from sleep. Maybe you can use it to make the guys with wireless keyboard envy you. I already envy you a little bit ;-)
And if you have a MacBook Pro since the NumLock has disappeared from F6 and it seems there's no alternative you may opt for the freeware KeyRemap4MacBook https://pqrs.org/macosx/keyremap4macbook/
It's very important in MakeMusic's Finale music notation program. It is used to clear out the notes in measures or the entire piece without eliminating the measures themselves.
In the calculator program (Dashboard or Today Widgets), use "delete" key to remove only one digit, use "clear" key to clear the number (all digits).