The standard bike mount isn’t strong enough to hold the light and the battery. The extension mount solves this by allowing the battery to be moved onto the bike frame.
With the exception of the standard mount, the PWR is the best bike light I’ve used.
The mount is of a high quality. Was hard to set up the light bracket straight on my handlebars but once it is done if doesnt move. The only downside was that my top tube is not the same diameter the whole length so the battery bracket can slip down.