Why do you use sugar when canning peaches or pears?
It somewhat depends on your definition of preserving.  

Sugar is definitely added for and too taste.  If you prefer less, then by all means use less using a light syrup rather than heavy, or possibly even none.  But, one of the purposes of sugar in canned fruit is for texture.  Canned fruit in light syrup will deteriorate in texture more quickly that in heavy syrup.  At least in theory, the sugar syrup will work through osmosis to replace some of the water in the fruit with sugar making it more firm especially after being subjected to the heat of canning.  Without this, the cells will break down and turn mushy.  

It seems counter intuitive to me at least, but to get this to work, the syrup needs to be higher in sugar than the fruit being canned, so very sweet peaches for instance would need a heavier syrup to firm when canned than a less sweet batch.  It is sad to risk over-powering that natural fresh flavor but is a cost of that type of preserving. 

In things like Jams and meat cures sugar is definitely used to help preserve, but those are different techniques.  In this case it helps preserve some of the quality, but not really against spoilage.