The four requirements of infantry...

[Der Kuenstler]

Don't forget to always outnumber your enemy, don't play fair.

This is another side benefit of flanking. If you send your main body up the middle, towards the flag, chances are that's where he will be sending his main body - the result is steel vs steel - often a bloody stalemate. However, if your main body goes somewhere unexpected (up the far side), you will increase your odds of meeting up with a small part of his force and get your desired many on few matchup.