Doubt regarding Threads

why we need threads as we already have processes in operating systems??

I found a good answer here
Process vs. Thread

Threads are light weight processes that divide main flow of control into multiple flows and each flow of control/thread will execute independently. Activity of the process in a system is represented by threads. The process that has multiple threads is called as multi threaded. Each threads has its own thread ID ( Data Type Integer), register, program counter, stack, error no. Threads can communicate using shared memory within same process.
There are different advantages of using threads to mange and maintain the subtask of applications. When we are using threads than less system resources are used for context switching and increased the throughput of application. Threads also simplify the structure of program. There is no special mechanism for communication between tasks.
Threads also have some disadvantages for example threads are not reusable as they are dependent on a process and cannot be separated from the process. Threads are not isolated as they don't have their own address space. The error cause by the thread can kill the entire process or program because that error affects the entire memory space of all threads use in that process or program. Due to the shared resources by the threads with in the process can also affect the whole process or program when a resource damage by the thread. For concurrent read and write access to the memory thread will required synchronizations. Data of the process can easily damage by the thread through data race because all the threads with in the process have write access to same piece of data.

AND one advantage is
# Context Switching Threads are very inexpensive to create and destroy, and they are inexpensive to represent. For example, they require space to store, the PC, the SP, and the general-purpose registers, but they do not require space to share memory information, Information about open files of I/O devices in use, etc. With so little context, it is much faster to switch between threads. In other words, it is relatively easier for a context switch using threads.
# Sharing Threads allow the sharing of a lot resources that cannot be shared in process, for example, sharing code section, data section, Operating System resources like open file etc......

In a few words the OS will manage the threads to run simultaneously many applications
but if you use only one thread in your application then your interface will be frozen while doing any intensive operation.
For example while scanning for files, or calculating crc values or write/read big files etc. if you don't use thread then the user will see a frozen interface and he has to wait for the operation to end, he can't even press a button to cancel.
Also using multiple threads you can do more than one job at any given moment inside your application.

Alex