You can probably charge your phone during the day at work, too.

A lot of people commute to and from work by bike because they love cycling but have busy lives, and this is a way for them to get more ride time in. These same folks might go out and spend the entire day biking through the countryside on a weekend. That's why you hear so much about battery power as a downside to using a phone. But it doesn't apply to everyone; pretty soon thousands of people will set out to ride 200 miles from Seattle to Portland but a lot of other people just want to get home from work.