Employers are in a unique position to offer group health plans. This used to be more relevant before Obamacare and is becoming relevant again. There is no equivalent to a group health plan one can buy individually.
The purpose of insurance is to amortize the cost of unpredictable things across time and population. Group plans are just another way to select higher quality (lower cost) people for the plan. Of course this means higher than average cost for programs like medicare.