Counting patterns in KSH

I was explaining the pattern matching in the shell earlier today, and used a very unfortunate example for explaining the "exactly one" pattern:

if [[ $var == @([0-9])% ]]; then...

I have (correctly) explained the above pattern as exactly one digit followed by a percent sign. One of the delegates asked me: why use the fancy characters? Wouldn't the following:

if [[ $var == [0-9]% ]]; then...

be the same? And if so, what's the point of the additional characters! Got me going for a moment! The answer is in the poorly chosen example. Although it works, one would never use the 'exact one' counting in relation to a single pattern specification.

The @(...) notation is used when you need to 'count' alternative patterns, as in:

if [[ $var == @(+|-)[0-9] ]]; then...

Without the @ character, the brackets needed for enveloping the alternative patterns would not work:

if [[ $var == (+|-)[0-9] ]]; then...
ksh: syntax error: '==' missing second argument

And without the brackets altogether:
if [[ $var == +|-[0-9] ]]; then...

gives:
ksh: syntax error: '|' unexpected operator/operand