Whilst it's not a legal requirement to purchase travel insurance, it is strongly recommended you do so, prior to departing on your trip. Neither the European Health Insurance Card (EHIC) or Global Health Insurance Card (GHIC) is a substitute for travel insurance – it is complementary to it.
The EHIC/GHIC may not cover you for all the medical costs that you can incur or for repatriation, lost baggage or cancellations.