Seasonal foods can be fresher and travel less distance to get to you, which helps with eating locally as well.
If you eat with the seasons of your geographical area, then conceivably the food is traveling less distance with less impact on the environment (carbon footprint) and supporting a more local economy. Eating seasonally often means eating locally. There is a nice map on Epicurious.com because, of course, seasons differ in each region.
Eating seasonally can be more affordable.
When food is in season, there is more of it and it probably travels less to get to you so the prices will likely be lower. Eating seasonally can save you money as well in this way.
Eating with the seasons gives you appreciation for savoring the foods you love.
If you eagerly await asparagus, cherry, or squash season, eat your favorite produce with every meal when it is at its peak, experiment with new recipes and feel some regret to see it go then you are eating with the seasons. With appreciation, we create a deeper respect for food, the way it is produced, and the way it tastes.
Eating seasonally can promote food safety.
Of course food can come into contact with contaminants at any point, but if you buy with the seasons from a more local source, it cuts down on the contact the food has with other locations, travel, storage conditions, hands and germs.
Seasonal foods foster a sense of community.
The more we learn about the foods that grow seasonally in our geographical location, the greater connection we have with farming and food practices in our area. If you take the time to visit farmers markets in your area, you can get to know the people who grow your food which fosters a sense of community.