If you have a medical condition then the normal practice used to be to contact your gp but that could take weeks to get the answer back so some insurers are using a nurse to ask the specific questions to speed it up.
Other alternative is to give all information you have on a condition at the time of proposing. i.e. medication taken, dates of last doc visits, type of investigations done, and send any copies of medical reports you may have. i.e. cholesterol/blood pressure readings.
The company may still need to go back to your gp for a report, depending on the information given or if a very high level of cover.