To be fair people use dog walkers for all sorts of reasons. Some people have changes of circumstance such as illness or changes to work practice. My neighbour for example has just started using a dog walker three days a week. She used to work from home, but a change in circumstance meant she had to seek external employment. She still walks the dog on the other four days and spends time with the dog but she also has two young children that make dog walking early in the morning or late at night impossible.
Some people live in apartments and like their dog to have a toilet stop in the middle of the day, particularly if the dog is very young or very old.
Some have an influx of work that makes them temporarily busy, or need to work longer hours one or two days a week.
Some just feel guilty that their dog is alone 8-9 hours a day while they are at work.
Some people also just have a lot of dogs.