Why do bread and butter pickle recipes call for onions, mustard seed, and celery seed? I recently made a jar of bread and butter pickles. I understood most of the rationale behind the ingredients and directions in the recipe - the cucumbers are salted to draw out water and break cell walls, then put into a highly saturated acid/sugar solution so that microbes present on the cucumbers would be destroyed by acid and osmotic pressure. Turmeric is not essential to preserving the pickles, but adds yellow color.

That leaves three ingredients I found in most recipes: onions, mustard seed, and celery seed. Why are these ingredients included in bread and butter pickle recipes? What would happen if they were left out?
Those are the flavorings.  

Just like dill pickles have dill in them, those are what give the flavor profile that people expect from bread &amp; butter pickles.