A good design helps your audience focus on the content itself instead of the design.
Creating Powerpoint can be painful for people who are not power users. Preparing for a presentation often means that you’re on a tight timeline.
While working in the office you would rather focus full attention on the presentation content and preparation than wasting time on creating presentation design from scratch.
Choosing right colors, fonts, graphics and changing design is time consuming process. It is easier to take a Powerpoint template as a starting point and edit it for your own needs.