There are countless reasons. The first ones that come to my mind are:

1. There are other errands that required one or more of the other members of the family (such as the husband or one or more of the kids), and instead of playing “musical children,” they decided to go as a family.
2. They work together with the shopping (seriously, especially with the older kids, you can send them out to different areas to get stuff, greatly speeding up the trip time).
3. They use it as a teaching tool for the older kids (math, budgeting, reading labels, etc).
4. It’s usually an enjoyable experience for the family (because sometimes, the sun, moon, and start align just right and everything blows up in your face, and this story just happens to be one such time).