[foepys]

It's also easier. When using "Sie" you need to use the infinitive form of the verb ("brauchen Sie etwas?", "werden Sie das so machen?") while when using "Du" you need to use the very special and complicated 2nd person form ("brauchst Du was?", "wirst Du das so machen?").

[eru]

Agreed in practice. I think in theory, it's not the infinitive form of the verb; just identical in most cases.

You can see that with 'to be': infinitive form is 'sein', but 'Sie sind'.