Greenhouse: Why?

To Extend the Growing season.  You can grow things in the greenhouse and prevent them from freezing, extending the temperature of the growing season for a month at each end.  It can facilitate multiple layers of frost protection, and if external heat sources are called for, such as an electric or diesel heater, a greenhouse can be a safer and more efficient option.

In places where the season starts late or when Fall comes early. For things growing in the greenhouse, you have less to fear from a sudden, unexpected frost.  If your peppers aren't completely done, they can be moved inside for that last few weeks of fruit production.

But keep in mind that low tunnels, that only enclose the garden bed itself, can be just as effective to extend the season.   They can also be easier to manage during the summer when they are no longer needed.  If  you are primarily interested in extending your growing season, then using low tunnels can be a better option.  Cold frames, low tunnels, and other temporary measures can be easier to work with, more manageable, and much less expensive. 

Also be aware that in some locations it isn't the temperature but the lack of daylight that limits the growth.

To extend the gardening season  The greenhouse doesn't just extend the "growing" season.  It extends the Gardening season as well.  The greenhouse is a place to think about gardening.  To plan, to innovate, to experiment.  

It gives you a place to garden in the dark evenings of late Fall and early Spring. It's a place to garden when its raining or windy, but you still want to be out in the garden.  The greenhouse provides a sanctuary where you can grow plants all year round and find peace and tranquility. It facilitates a significant jump on the early season just by putting you in the frame of mind to garden.  And it's available any time of the year.

To give yourself a place to work.  This is one of the more important functions of the greenhouse.  The greenhouse can be a general purpose workspace to support the rest of your garden.  It is a dedicated space for gardening, separate from the rest of the yard.  It can make gardening less reactive, more intentional.  Less dependent on the weather.  Less of an "outside" activity, when necessary.  This is when gardening becomes part of the gardener's routine and their lifestyle.

It allows you to keep working in the garden even when its dark or raining. After work, after dinner, you still have a place to focus on your garden. You don't have to retreat indoors and abandon your plants.  It can also be heated, at least temporarily, in the winter.  It can be a place to work on all the things that don't directly relate to actually handling plants.

A place to start seedlings.  The greenhouse represents the first stage in the process of gardening:  Here is where the seeds are sown.  Then transplanted to pots. Then moved out into the raised beds.

One of the biggest factors in starting seedlings is a space to work.  You need to have a workbench or a potting bench to fill your trays, manipulate the seeds and work on one seed tray at a time.

Next, you need a place for newly seeded flats to germinate in the full sun.  This will require a good amount of space to lay everything out.  You should allow 4 weeks for a freshly sown seed tray to reach the stage where they have true leaves and are ready for the shock of transplanting. 

Eventually, you need to transfer those seedlings to larger 3-4" pots, that will continue to mature.  This will get them ready to be planted out in the garden at the appropriate time.  Typically, that's another 4 weeks of growing on.  And its important to note that you need to be able to hold them at the pot stage, because conditions vary from year to year and you'll need flexibility as to your planting out date.  

While this is happening, you'll need to be preparing for the next round of succession partners to begin the journey.

Overwintering.  The greenhouse can be a place where you move sensitive plants indoors to escape the frost and snow.  In an unheated space, plants are still susceptible to deep sustained freezes.  But a greenhouse can keep dormant plants from being killed by lower temperatures that they wouldn't survive outside in the elements with desiccating winds and freezing rain.