They explain the reasoning for this move:

The team decided to base the unified client on their web player's codebase, with CEF hosting it on the desktop. The previous architecture of the desktop client Every 'page' in the client was built as a standalone 'app' to run inside its iframe. It was a native Windows and Mac application that used the Chromium Embedded Framework (CEF) to display a web-based user interface.

The engineers designed the previous desktop client to foster autonomy, allowing multiple teams to own the development and maintenance of the features. Since our Web Player is continuously deployed, we could ship and test with real users every change made towards our final goal. We settled on a bold solution: focus on iterating on top of the existing Web Player codebase until it reached a Desktop-grade feature set. We knew we were embarking on a long-term project, so our biggest priority was to de-risk delivery and avoid trapping ourselves into a big bang rewrite. Spotify's team explains their strategy for approaching the code convergence project: Spotify continuously conducts user tests to meet the desktop and web player users' needs. As a result, their development speed increased. The new architecture made UI coding simpler and easier to understand as developers.

The project permitted Spotify to unify the user experience and visual design across platforms and make it consistent with the rest of the Spotify ecosystem. Spotify reused the same code in multiple clients, which allowed them to write the code once and reap the benefits in various places. Spotify had four main goals with which they evaluated the project's success. Now Spotify created one maintainable codebase for both, resulting in an improved development cadence. Due to having to implement many features twice, Spotify's engineers were not shipping new features at the pace they wanted. At the end of 2018, Spotify's team was the owner of a recently built web player, as well as a separate, rich, full-featured desktop client. Spotify recently published a detailed account of its journey towards a unified codebase for its web and desktop clients.

Spotify webapp