I never really thought I'd want to grow much corn until today.
Too bad the seed already sold out for now.
Quick explanation of heirloom seeds and their importance? Ok.
In general, the best heirloom seeds are open-pollinated (by birds, bees, other insects, wind, natural means), and not hybrids which would not occur naturally. Nor are they genetically modified in any way. Because they're not hybrids they will sow true -- which means that if you properly grow, collect, store and plant an heirloom seed it will be true to it's parent plant.
This is not true for, as a popular example, a hybrid tomato plant. If you saved the seed of a hybrid tomato odds are if you planted the saved seed you would get a cherry tomato (per my experience) or at least one of the parent varieties used to create the hybrid -- not the hybrid itself. Hybrids exist for a variety of reasons, mostly because they express the beneficial traits of the parent varieties. I personally consider hybrids a hard sell because you must buy or renew each year -- not very sustainable environmentally or self-sufficient.
Heirlooms are important because of their genetic consistency and stability. Plus they tend to pretty much always exhibit prettier and tastier qualities over the hybrids and GMOs. There are also so, so, so many kinds. Which has made for a lot of really neat seed saving movements, banks and libraries. Besides being cool to collect -- they are of extremely understated importance to maintaining diversity in our food crops which, in turn, invests in the hopes that we will always have something edible to grow.
Plus a lot of banks and libraries are regionally-focused which is a sustainability measure as well so that we spend less resources (water, greenhouse heating, etc) trying to grow things that aren't native or very well acclimated locally.
Did I mention they're prettier?
New recommended reading about heirloom seeds for sustainability by the director of Seeds Trust, Bill McDorman.