Heritage varieties were seeds selected by our ancestors that did best in their own soils and growing areas. What did well in Scotland may not do so well down in Kent, etc. same with growing conditions - too acidic, too alkaline, etc. So our ancestors selected what worked for them and passed down the seeds. Its not to say that the seeds won't grow in other areas, they will, but they just won't be as hardy or prolific as in their gathered area.
Another point about heirlooms/heritage, is that they were also selected for various area pests and disease problems as more resistant. Again, not to say that they were totally pest or disease free, but that they did better than other varieties for that area.
Also, maturity times varied greatly. Scotland might like faster maturing toms while Devon might not need anything so fast. My mother lives 30 minutes east of me, but she's in a much higher elevation and must grow only 60 day toms or the frosts get them. I can grow any tom variety I want (which drives her bonkers
). Again, the preferred variety was grown and from that the earliest maturing toms would have been selected more closely, etc.
The extent of hardiness, colours, shapes, etc are a wonderful 'mine' to work with which is why hybrid breeders still need them to breed future generations. And should the day happen when hybrids lose their original 'parent' plants, the rest of us still have lots of choice left. Plus, often hybrids must sacrifice flavour for perhaps better shipping, etc. Keep in mind that hybrids were originally developed for the commercial, not the domestic, trade: delayed ripening, better shipping, uniform colouring or sizes, etc. Home growers don't need these but the hybridizers found an extra market for their products when home growers thought they were getting something better. Must admit that Sungold toms are a definite exception (yummm).
The biggest problem with heirlooms is the huge range of choice in any veg.
There are hundreds of choices alone in toms, and still more coming out of forgotten drawers, etc.