First there's the obvious, and quite significant, tax benefits to employer provided healthcare over private purchased healthcare. In the former case, neither the employer nor the employee pay payroll taxes on their contributions and the employee portion is also exempted from income tax (the employer also doesn't pay income tax on its contribution, but it wouldn't for cash salary either).

Second, more subtly, the group market faces much more favorable underwriting than the individual market. Even if non-employer group health insurance were allowed (which it isn't anymore under ACA) such groups are always subject to self-selection bias and thus are unlikely to be underwritten as as favorably. However, this point is somewhat tricky because there are both cross-subsidies and gross economic surpluses, which can be hard to untangle.