In the business, we used to say that cables stretch. It's not true, but they behave as if they stretch. Housings compress, and they also settle into their cable stops. This results in cables becoming loose. It's not a big deal unless it's annoying. You may want your derailleur to come out of high gear as soon as you move pull the lever. Or maybe the lever pulls a lot of cable to get to low gear but you run out of cable. A barrel adjuster effectively lengthens the housing which then effectively shortens the cable.