Cold frames are still widely used in Britain as a half way house between a greenhouse and a planting out. When you have reared your seedlings in the greenhouse, if you plant them straight into the garden in April May or June time, they will be be shocked by the cold particularly at night and this will literally stunt their growth. They may not grow at all for weeks after such a shock and that is why cold frames are used in conjunctio with your greenhouse. The purpose of a cold frame is to create shelter s that you can harden-off¹ your plants.
First place them in the cold frame with the lids closed, then day by day have the vents more and more open in the day (replace or close at night)until eventually, you remove the lids completely for the last day or two. Hardening off should take 7-10 days for best results. Polycarbonate cold frames are cheap and lightweight so easy to move about the garden (but vulnerable to wind - do make sure you anchor them or weigh them down) But nothing beats a glass cold frame (preferably with toughened glass).
The best sort have lids that both prop open and or slide across and be removable for flexibility.