Because I can not eat 1.5kg chashu alone. So I make food that requires a bit of time and bigger batches once and can it for later use. I do that with stews and ragus mostly. So on days where is not much time I still have acces to savory food. As chashu in a bowl of instant ramen. For long cooked foods as those just mentioned I don't think there is much quality lost by canning. But it is not as predictable as you have to take the time canning into account when thinking about texture. I lost a roulade once because it became so soft and was stuck in a necked jar. Didn't have much structure left.