The answer depends on the type of seed you are looking to germinate, and plants you are looking to grow. Different plants require different temperatures for proper germination. Some cold loving plants can literally be grown year round in a greenhouse. Some of the cold loving crops are mentioned here;
When using a greenhouse to start seed, you'll need to determine the proper germination temperature, and approximate time of germination. Once you know those two things you can begin planting when conditions in the greenhouse are right for germination. Generally, seed companies give recommendations as to when you should start the seed for any given plant.
The idea behind starting early in the greenhouse is that you have established plants ready to go in the ground when last frost has passed. We generally start seed a month before last frost, and after hardening them off, put them in the garden after danger of frost has passed. The USDA plant hardiness zone map will give you an estimation of last frost dates in your area.
A greenhouse is also useful for growing things that you normally wouldn't be able to grow in your climate zone. For instance, tropical plants in the northern zones.