Update: You may now use "universal" instead of isomorphic (see this post).
The difference with regular SPA is that any page you access directly will be initially server-side rendered. You can check that by accessing directly a page and displaying its source.
Even if server-side rendered, the SPA keeps working as usual (front-end router, template rendering ...).
But ... why bother about that ...?
I won't go into specifics but here are two main upsides :
TLDR; from stackoverflow
Learn more about that and this project on my blog post.