The baking soda or stabilizer act as the buffer. The solution will become acidic overtime and will kill the yeast solution. By adding this, you'll somewhat prolong the solution's life.
I don't have much success using Fleishmann (spelling?) yeast. I did everything as told to activate the yeast first, etc. It takes long before the first bubble comes out and the production amount drops after 5-7 days. I switched to Redstar and get very good result. The first bubble comes out in 20-30 minutes or less, more pressure, constant and last much longer. (Both are regular active dry yeast and not super). It even starts well without proper yeast activation. I'm not sure whether my tap water has something to do with those results because I heard more people here use Fleishman.