Making jam usually requires pectin which is extracted from fruit. If the traditional purpose of making jam is to preserve fruit, why would it be a good idea to extract the pectin from one fruit to preserve another. This seems circular.
The "paradox" here is a result of an oversimplified explanation of reality. The crux is in
the traditional purpose of making jam is to preserve fruit
For the addition of pectin to make no sense, you have to make two assumptions: 1) that the only reason is to have access to fruit in winter (which is what I think you mean as "to preserve fruit"), and 2) that no matter what you do in your life, you have to always be efficient, as in not wasting resources and trying to get the highest amount of output you can.
I would argue that both are untrue. The efficiency argument is highly prevalent in our current society, but it is not an absolute imperative. And the idea that people do things for a single purpose, and it is the same purpose for all people doing it, is simply untrue for most situations in life.
There is not "the purpose" of making jam, there are many different purposes. Here is an incomplete list in no particular order:
- Having access to some form of fruit in winter
- Having a sweet bread spread
- Having all the ingredients for that pie recipe you absolutely love
- Being proud of something you made yourself
- Having a nice gift with a personal touch for people who can afford to buy what they need anyway
- Feeling connected to the traditions of your culture
- Learning something new
- Having more personal connection to your food
- Having an activity in which you can involve your whole family
- Doing something about these 17 kg of ripe cherries on your tree that are all ripe in the same week
People make jam for some combination of these reasons, with different people giving different weight to each of them.
Sure, if you are thinking of a medieval peasant in Western Europe who is dependent on subsistence farming, both your assumptions are likely to be true, and it would make little sense to add pectin to jam. But that farmer wouldn't have had access to powdered pectin anyway, nor to many of the kinds of fruit which need pectin for canning anyway. I'm pretty sure they must have made their jams mostly without added pectin. But modern food has developed under completely different conditions, and there is no barrier to pectin becoming part of jams.
The pectin is not required to preserve the fruit. You can make perfectly safe jam or marmalade with just fruit and sugar, it just might be a bit runnier than you would prefer. The pectin is added to thicken the jam to a desired texture, especially if you're preserving a fruit that is naturally low in pectin. In more traditional cooking, one could easily make a "mixed" jam by adding for example cooking apples or orange peels (high in pectin) to a strawberry jam (low in pectin) to get a thicker consistency.