There are many pros and cons for each choice. Most of these systems also give the consumer a lot of free software with their system purchase. Name brands also have support networks for the numerous questions that come up with new computer setup and use. That means things like sound cards, modems, network cards, video cards are built into the mother board. This makes upgrading these items difficult or even impossible.
Some vendors even have proprietary memory chips.
You can not buy the cheaper generic memory to upgrade. Some vendors ask for your visa card before they will answer your questions. Your new hard drive will come loaded up with programs that you may not use and that just take up space. A CBS will have a mother board that allows you to upgrade internal components. The hard drive comes with only the operating system so you choose what applications you want to install.