You can, for example, introduce new operators very easily:

multi sub prefix:<foo> (Str $a) {
     return $a eq "foo" ?? "bar" !! "foo";
}
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This will define an unary prefix operator that takes one string as an argument. When somebody writes

foo "foo";
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You can't know if that's a method call or a call to an operator. You'd think it makes no difference - but wait until precedence comes into the play:

BEGIN {
    eval 
    "
multi * prefix:<foo> (Str $a) is tighter(&infix:<~>) {
     return $a eq 'foo' ?? 'bar' !! 'foo';
    ";
}

# and later in the code:
foo "fo" ~ "o";
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If that foo parsed as a sub call the precedence is lower than that of the concatenation ~, and the other way round if it's parsed as an operator.

This is quite a complicated example, but there are much easier ones with "real" macros. (But since no implemenation really implements macros by now, I don't really know much about them).