Imo it's more fun and interesting to experience different universities (but I'm someone who has gone to different unis for undergrad, Masters and PhD).
Pros of same uni:
---You're already familiar with the university facilities, culture (the way things work there), environment, admin team, local area, familiar faces. So you don't have to waste time relearning these things in a new place. And the comfort of familiarity means all your energies can focus on the PhD.
---You can make a very informed decision of whether you get on well with the academics/supervisors to help you thrive on your project.
---There may be financial benefits, like alumni discounts, preference for alumni for scholarships (maybe you know the academics well and they work hard to help you get it), and you have the local knowledge of the area to choose cheaper options in day-to-day life.
Cons of same uni:
---Potentially boring and missing out to stay in the same uni for 7+ years, never able to experience life at a different institution and city - variety is the spice of life. You miss out on the expertise, experience, methodologies, and strengths of different academics and resources at different unis.
---Being able to adapt to new places is an important skill.
---It can bring whole new perspectives to your work and your life in general to attend different universities, which is inspiring and motivating.