Why the concept of Single Thread Model interface is used?

Safety and security are always the priority for any company. Thus, the single Thread Model interface introduces for this reason. The main purpose of this concept is to check whether two or more threads are executing at the same time. Like many threads may be penetrated by many requests, so to execute them in a manner, it is used. But in servlet 2.3, this concept is disapproved as it can stop the multithreading method.