Hi Veena,
Usually Companies keep soem of the workers on the payrolls of third party to reduce the costs which are incurred to the company.
If the worker is on the pay rolls of the company, he will be eligible for all the welfare activities which the company do and in turn it will cost the company. They can even pay less amount in terms of salaries to the workers if they are on the pay rolls of the third party.
Outsourcing the workers is a great advantage to the company. Usually, Companies make a worker Permanent, if he does a great work and this permanent employement would make him to stay with the company for a longer time period.
Regards,
Gowtam