Lower server requirements. A thick client server does not require as high a level of performance as a thin client server (since the thick clients themselves do much of the application processing). This results in drastically cheaper servers.
Working offline. Thick clients have advantages in that a constant connection to the central server is often not required.
Better multimedia performance. Thick clients have advantages in multimedia-rich applications that would be bandwidth intensive if fully served. For example, thick clients are well suited for video gaming.
More flexibility. On some operating systems software products are designed for personal computers that have their own local resources. Running this software in a thin client environment can be difficult.
Using existing infrastructure. As many people now have very fast local PCs, they already have the infrastructure to run thick clients at no extra cost.
Higher server capacity. The more work that is carried out by the client, the less the server needs to do, increasing the number of users each server can support.