Answer:
Bootloader is the program that helps in the revitalizing the micro-controller of the operating system .It makes the operating system self programmable.It's main purpose is loading the tasks or operations to ensure the working interface between software and hardware whenever the system gets booted up.
It is also known as the bootstrap loader.It is necessary and common because it is the first mechanism that works whenever the system gets initialized or booted. Without this feature the reloading of the programs will not be possible.