Spare battery, great for users who use their phone extensively whilst out and about. Lets you keep talking and makes sure you don't miss important phone calls. Simply swap out with your original battery when you run low on power.
Fuming after vodafone didn't sent me a battery with my new phone I went out a bought one, really fast shipping arrived before I expected ready for the weekend. The battery life far excels my mates official HTC one thanks.
It is a very good price and so far it seems to hold a charge well and last a fair bit longer than my original HTC battery. However, it will be interesting to see what it is like in 6 or 12 months time.
Added later. I have now had it a month and it is lasting only a few hours on stand by. Not what I hoped. It was a low price - and now I see why.