Sandboxing isolates applications so that malware doesn't spread beyond the insecure webpage to other parts of your computer. It's an additional layer of protection to further guard against malicious pages that try to hijack your computer or steal your private information. Plug-ins are a huge security problem for browser vendors, and this is Google's attempt to contain them.
The latest beta also adds a feature called Chrome Instant, which you can enable to load web pages that you frequently visit as soon as you start typing a URL. If your default search engine supports it, search results will also appear instantly as you type in queries.