I was reading a recipe from a cookbook that focuses on Bombay cuisine, and there is one for a tomato-chilli jam in it, which reads:

1.Blitz the tomatoes using a jug or stick blender, in batches if necessary, until you have a coarse purée. Add all of the remaining ingredients except the sugar, and blend until well combined.

2.Pour the blended mix into a non-reactive cooking pot and add the sugar. Bring to a gentle simmer and cook over a low heat until reduced to a thick consistency; this should take about 30 minutes.

The part that I am confused about is why it would specify to withhold the sugar from the blending process, and then add it in immediately after and cook? Does sugar somehow interact with tomatoes or spices in a particular way during blending?

Note: The 'remaining ingredients' referred to in step 1 are rice vinegar, garlic, ginger, and green chillies (all finely chopped)