our creates a lexical scoped name for a global variable.

Your example shows that you can only use local on global variables

Well, it more on anything that isn't a lexical variable. You can localize array elements for instance, even if the array itself is a lexical variable.

So why use my anyway

Because my creates lexically scoped variables. Variables that aren't visible outside of the block that declares them. local creates temporary values, which are only reset if a block gets exited.

The right question should be "why use local?".