The function of the "Alt" keys hasn't changed much, they are still used for the same purpose as they did when they were first introduced.
They allow multiple combinations of keys in a sequence to...
They are used for advanced Dual key usage, For example on a windows PC you hold down the alt key and press the F4 key, the window that is on top will be closed. If you carry on with this sequence...
This depents on the font set you are using. If you are using "Wingdings" you can hold the alt key, type "34", then release alt. I also recommend the Character Map program in Windows, Start-> Run...