Some companies offer life or health insurances for their foreign workers, some cover only hospitalization. Larger companies have agreements with private hospitals or doctors to care for their workers. You should check terms and conditions with your employer and then discuss whether private insurance is helpful.
Since public health care is almost completely free of charge in Sri Lanka people sometimes complain about standards or long waiting periods. This is why those who can afford it tend to pay for private institutions. In fact they are comparatively cheap in Sri Lanka, so medical tourism has become a popular topic over the last few years. Public hospitals are open to both locals and foreign people, with the exception that foreigners have to pay for the medicine. Nevertheless they sometimes expect you to donate an appropriate amount of money.
The standards and technologies of private hospitals can be higher than in public ones. Nevertheless, public hospitals in Colombo have good standards while private rural hospitals may not. So do not expect wonders just because it is private and inform yourself before you choose a place.
In general you should check if your insurance covers transportation to another country since Sri Lanka’s hospitals may not be able to treat everything. Serious issues should be treated in private hospitals so it really makes sense to have comprehensive health insurance.