Think of cold frame (CF) as a mini unheated greenhouse.
== you can use it to germinate seeds to be transplanted. For example, you can direct sow lettuce, broccoli, etc but it will take quite a while to germinate that way. But in cold frame they will germinate in a fraction of the time. This will give you a head start.
I will not start such crops indoors, under light when I have a CF. Today I am going to start few more. Forecasts look good.

== You can use CF to shelter seedlings, BOTH during hardening off and later, IF and When there is danger of frost or it is going to be a chilli night.

An example of CF Use:
I have already sown seeds of lettuce and few other cold crop in there. This gives me a pretty good idea as when I should start more. Of, course, it depend on your climate (sunshine) and location of your cold frame. More sun translate to much early use of CF. As I mentioned, it is a mini unheated greenhouse.

== you can keep certain smaller seedlings in CF for a while early spring, to let them grow bigger, before transplanting. This will depend on how big and how tall your CF is. Slightly taller ones can accommodate bigger pots and plants.

CAUTION: In a hot sunny day, CF can get real HOT inside. Therefore you have to vent it (by partially leaving the cover open). Otherwise you may cook your plants. This can happen in greenhouse too.