There are quite a few problems with Tropicana Field, depending on who one asks. While the Tampa Bay Rays home stadium possesses a few delightful quirks, it is almost universally considered to be outdated. Then, there is the issue of being able to access the Trop itself. Given its location, it is not exactly the easiest venue to watch a baseball game at.
The idea is that a new stadium will correct those issues. The Rays would then be able to actually draw a viable crowd, increasing their payroll, and keep their franchise players. At least, that is the theory.