When you have 2, you have extra power during time consuming jobs but you do not need to purchase a new battery with every tool you buy.
Once you have purchased 1 battery, you can combine it with any HENX tool body.
Also, batteries last longer if they are used regularly. This means, you can enjoy your battery longer! And not to forget: it is a lot better for the environment too to use battery power instead of gasoline garden tools.