The battery in the Lumia 640 is a decent size as it is at 2500mAh, likewise with the 3000mAh in the Lumia 640 XL. But, both also have the added bonus of being removable. You shouldn't face too much trouble getting through even a relatively strenuous day, but it's handy for some to know that you can swap it out at any time.
With the XL in particular, Microsoft is pushing as a great device for business, and in that kind of arena a battery that can churn on all day is important.