Because nobody likes to learn everything at a desk! And nobody should have to, especially when learning skills in a variety of contexts can be very beneficial for learning.
According to a classic psychology experiment, you are more likely to remember new information if you are tested in the place where you learned the material. So scuba divers who were taught information on land recalled the info better when they were tested again on dry land and the divers who were taught under water recalled the info more accurately when tested under the deep blue sea.
Now if you relate this back to teaching your child, if you only teach them things at the table, then the chances are that they will be less likely to perform this skill in other contexts.