The application should open faster; it's usually cheaper to read one large file than several smaller ones. This may sound negligible until you have hundreds of textures, and then it becomes pretty noticeable—even more so over the network: HTTP requests add sizable overhead, which may become a deal breaker on mobile devices with constrained connectivity.

Rendering is also subtly quicker when there's no need to rebind textures. Using a texture map effectively means that only the texture coordinates are affected by any change that would otherwise lead to switching textures.

The previous point also makes atlases more suitable for use in situations when there's a single large model, such as our GLSL-based renderer...