Final answer:
Calling start() on a thread creates a new call stack and enables concurrency, while calling run() directly executes the method in the current thread, which does not start a new thread and thus misses out on the benefits of multithreading. The start() method is crucial for achieving parallel execution and improving performance in multi-threaded applications.
Step-by-step explanation:
The start() method in threading is essential because it does not just call the run() method; it creates a separate call stack for the thread. When you directly call the run() method, it is executed within the current call stack rather than starting a new thread. This means that if you called run() directly, you would not be starting a new thread but rather just executing the method in the current thread, which defeats the purpose of concurrency and the benefits of multithreading. The issues this would cause include lack of concurrent execution, potential blocking of the user interface, and inability to take advantage of multiple processors.
Threads are designed to run in parallel, allowing the program to perform multiple operations simultaneously. The start() method enables this by asking the Java Virtual Machine (JVM) to create a new thread, which then calls the run() method. This separation between start() and run() is what allows programs to improve performance through concurrency.