Heirloom varieties have been kept true to type over the years by breeding in isolation from other varieties of the same species. The genetics of these cultivars have been developed and saved because they were delicious, hardy, or valuable in other ways. They are important vessels of genetic diversity, but they may not thrive under your conditions.
Heirlooms are inbred varieties, adapted to a specific place and time. What grew well for someone 100 years ago in a far away place, probably isn't optimal for your garden today.
Do you struggle to keep plants healthy? Add fertilizers, battle insects, or protect your plants from heat or cold? Saving seed and creating varieties that are adapted to your specific climate, pest pressures, and habits as a gardener will make your crops more resilient than any seed you can buy. These stronger plants will require less work, and fewer inputs such as fertilizer, pesticides, and water.
Locally adapted plants are better for you, and for the environment.