A template in Builder is an easy way to copy a piece of content and paste it many places and make modifications. A symbol is a way to reuse the same piece of content in many places such that editing the symbol in one place updates all places it’s been used.
Templates are great as a default for saving things that you want to use again. Symbols are great for content that you want to keep up to date on many pages without updating every single page every time individually.
See the video below for a more detailed walkthrough of the differences: