A problem when designing a digital experience is limited screen real estate. Multiple stakeholders trying to cram as much content into the page while designers fight to keep it aesthetic. The goal is to show the most relevant content in the most accessible space but that's different for every visitor. Personalization allows you to tailor experiences for different audiences, allowing you to reorder and optimize layouts for their unique needs.
The common use case is to simply have different variations of a page for different, mutually exclusive, audience personas. For example, a solutions page might be optimized for the different verticals the company caters to. In these cases, the easiest way to personalize would be to create a default page and tweak it for the various personas.