The Shared tab in the plugin builder does exactly what it says on the tin; any code inside is processed before any View, allowing you to keep logic and templates in one place, and keep layouts focused on the look of things.
DRY Code
DRY (Don't Repeat Yourself) helps keep things organized and reduce the chance of copy/paste mistakes when developing or improving your plugins.
It's a best-practice to put Liquid logic in Shared if it's preprocessing Your Variables. If you put "A" in Shared and then "B" in your Full view, then plugin would display "AB". This is equivalent to copy/pasting all the Share content into each view (exhausting!).
Templates
Shared doesn't have to only include logic, it can also include a Template, which you can pass key:value pairs to render content dynamically, while still limiting duplication.
For more info, check out our plugin docs.
TIP: When using a template, Your Variables are not automatically included, you have to pass them in. This is most commonly a "gotcha" when you need the trmnl variable's data, your render could look like:
{% render "my_template", trmnl: trmnl %}