Two things I've found beneficial are:

In a single page app, your client routes (representing individual views) don't have to map directly to your server side routes, which gives you the ability to have pages/views that don't require a server roundtrip.

You can also cache frequently used data on the client and use them on multiple "pages" as you navigate through your client side routes/pages, loading "page" specific data from your API as needed.

So in general it gives you more tools/options for building a snappier app.