In Finland, there's not really a compulsory medical coverage by employers. The only compulsory part is planning and processes to prevent work-related health hazards and harms, and to protect and advance workplace safety, ability to work and health. (See law 2001/1383)
So, some paperwork is mandatory, no actual health care.
The reason many employers voluntarily provide an actual health insurance (or simply reimburse private-sector medical costs directly) is that it is one of the few tax-free benefits that employers can give to people, so it works efficiently as an additional compensation. And of course, because employee absences are costly, it makes good sense to try to keep them in good health.
The public sector health care does practically nothing in terms of preventive health care for people between 25-65 and in employment (except for maternity-related things). This is left to employers.