A big one is that there is a tax deduction for employer provided health insurance. So offering a decent health plan is more compensation than just handing over the cost of the plan (which would be taxed).

As far as how employer coverage can drive up individual costs, there are a couple of factors. One is that people with good health plans often don't pay attention to how much they cost, and often don't have much of a deductible, so they just use the medical system without paying too much attention to costs. That at least has the potential to drive medical prices up.

The other is that people in the group insurance market tend to be somewhat healthier than the individual market. So the pool for individual insurance is more expensive to insure. Setting aside discussions about whether it is fair or not, it does result in higher costs for the individual market.