Going a little off-topic here but I have never understood the model of having health care provided by an employer. In Australia we all pay for our own private h

Going a little off-topic here but I have never understood the model of having health care provided by an employer. In Australia we all pay for our own private health care (or choose not to). For one, I just can't understand how health care is in any way related to employment. More importantly, what happens if you get sick and then change jobs (and hence change health care providers)? How do pre-existing conditions work in this model? Is this just a way to de-incentivise job changing?