Sometimes it is simply cheaper in the long run to buy another tool with a battery and charger then to buy replacement or additional batteries for your yard tools.
Take this example. I have the excellent and affordable WEN battery chainsaw which is perfect for a homeowner with occasional tree trimming or firewood preparation jobs. The battery lasts a long time and the saw cuts well, especially with a fresh or recently sharpened blade. I recommend having a three-pack of replacement chains on hand for big jobs. When you buy the three-pack of Oregon chainsaw blades, each chainsaw chain blade is less than $10.
So when I had a big tree-cutting job I felt the need to have another battery pack on hand. Replacement batteries cost $70 so I figured, why not get the Wen battery-powered leaf blower which uses the same battery? For $120 I get the leaf blower, the battery and another charger.
This leaf blower is going to be super handy for quick jobs like cleaning off the deck. No need to haul out the corded one or mess with gas and oil like those noisy, polluting fossil fuel-powered leaf blowers. My neighbor across the pond has one of those noisy industrial backpack leaf blowers and I want to strangle him every time he fires it up.