Well when you first bite into that home grown heirloom tomato you will understand an answer to that question because it will taste like a tomato not the red hard thing you generally buy in a supermarket.
An heirloom seed is a seed that will produce what it came from and it will reproduce. A hybrid has at least two different “parents” and it does not reproduce true to type.
The genetic material inside a uniquely colored pole bean from Spain, say, or a rare, purple potato catalogued by ancient Peruvians – or the famous, two pound tomato grown by West Virginia’s legendary Mortgage Lifter “Charlie” – these are treasures passed down from history for anyone and everyone who cares to participate in their legacy. You don’t need special permission to grow, sell or trade these varieties. You don’t need large acreage, an advanced degree, or lots of time or money to save their seed. In fact, for many basic crops, like peas, beans and tomatoes, all you need is the will, the knowledge and the timing to select, gather, dry and store what nature so conveniently provides.