There are many different reasons for using a robot but the central reason for most applications is to eliminate a human operator. The most obvious reason is:
To save labor and reduce cost.
Other classes of applications concern the product:
Human is bad for the product for example semiconductor handling.
Within this class are other reasons for using robots for example food handling, pharmaceuticals, etc.
Product is bad for the human for example radioactive product.
Within the above are other reasons for using robots for example robots can be used to replace human operators where the dangers are:
Repetitive strain syndrome.
Working with machinery that is dangerous for example presses, winders.
Working with materials which might be harmful in the short or long term.