[iraldir]

An isomorphic app is basically a Single Page Application (think Angular / React / Ember application) with the special ability to be rendered server side. The interest vs a simple SPA is that on the first page load, instead of loading just javascript, and having to wait for XHR request and JavaScript rendering to draw the data, you get the page properly rendered in the first place. But when you navigate to go to a different page, you use javascript to repopulate the data. It's the best of both world for the users. But for the developers it usually means extra work.

[MasterScrat]

Also it's better from an SEO point of view, since depending on how you implement your SLA Google may be unable to read it.