Our winters are long with risks of late frost in Spring and early frost in late Summer. Confused? I don't blame you! Know this. We get frost both early and late in the season and it kills plants. I learned this the very hardest way possible (on both ends of the growing season) as a new "country gardener". Temperatures are often several degrees colder than in the city, so we decided to fight back by building permanent season extension.
Building a greenhouse has helped us to grow more food largely because we can get those crops needing more "days to maturity" planted early enough to have them reach harvest before a killing frost. I use sawhorses with plywood and old hollow core doors to create temporary seeding tables which are easily taken down when those plants are hardened off and planted outside. This then frees up the north growing beds in the greenhouse (under the makeshift tables). A greenhouse also gives us some degree of late season protection for tender crops which might freeze before ripening if grown outside.
As with any system, diversity is the key to resiliency. I never plant all of a tender crop in the greenhouse (nor would I plant all of a tender crop in the main garden). In some years, we get lucky and can harvest heat lovers from the main gardens before a killing frost (making the greenhouse redundant), but often, we get only a portion of them harvested before they freeze solid. In those "frosty" years, the greenhouse saves us from a long winter without tomato sauce (a fate I'd rather not endure again).
Row covers occasionally come into my season extension plan although I must be honest - I don't like them. They are a lot of work to put them in place alone and are a huge hassle to remove and reposition (for weeding, succession seeding and harvesting). I don't generally like any system that I have to fight and row covers certainly can be difficult. The easiest way to keep them in place (after MUCH experimentation) is to use lengths of rebar (simply laid end to end along each long outer edge). I can roll them off easily to check underneath and roll them back on to secure it again without pegging and tucking. If you live in a windy location, I don't recommend using them. My main garden is very protected from the wind, so it's not usually an issue for me in this location but I wouldn't use them elsewhere on the property.