Best use cases for snippets?

Anything that you typically repeat/reuse in your content.
Ive seen them most commonly used as footers, such as company contact details, at the end of an email or landing page.
They are very useful when combined with dynamic content too

One way we use snippets for our footers for emails on our international offices. Each footer has brick and morter address and policies links for that country. Snippets help with that. Then moved to using segments with snippets.  

We have different footers for our different publication emails. 
We also use it for different content areas that we easily want to swap out.
For sponsor listing with logos by event type.  
For different multiple signatures on the email.

I would agree with Cathal for the use of snippets with dynamic content. The ability of snippets to replace editable sections and their use with segmentations and tokens is the an effective way to leverage dynamic content.