1. It sandboxes resource usage, preventing Slack from being a permanent resource hog. Is your Slack tab running hot on Chrome? Just kill it for now. There are many browser extensions for managing resource usage per tab.
2. Closing the Slack browser tab means no disruptive notifications. Sure, you can snooze Slack @-mentions, etc., but that's too much of "working hard to make the tool work."
3. Better workflow (seriously) Perhaps this is just me (or my function as a marketing person at a startup), but I work almost entirely inside the browser (GMail, Google Docs, various sites for research, SaaS apps). Using Slack as a standalone desktop app means I have to focus away from the browser. Using Slack as a browser tab means I can treat it just one of several web apps I use regularly.
4. Bonus point: No need to update your Slack client =)