There are three main reasons why you may want to buy a motherboard: to replace your computer's motherboard to improve its performance, to replace it because it is failing or has broken down permanently, or to mount your own PC.
Either way, you should decide between opting for a long-term model or one that meets short-term requirements. As you can imagine, the former will be more expensive but also offer the possibility of improving or expanding some of the components in the future.