yeast with baking soda?

yeast with baking soda?

Hi there -

Just wondered if anyone knew why baking soda added to recipe below if
there is already starter used to raise the bread. Also, why yeast added
as last ingredient? I thought it was redundant and changed sourdough
flavor. Guess I have to bake it and find out. Thanks for help!

SOURDOUGH RAISIN BREAD
ingredients
€ 1 cup active sourdough starter
€ 1/2 cup raisins
€ 3/4 cup lukewarm/cool water
€ 3 cups bread flour
€ 1/8 teaspoon baking soda
€ 1 tablespoon sugar
€ 1/2 teaspoon salt
€ 1 teaspoon cinnamon
€ 1 tablespoon oil
€ 1 1/4 teaspoons yeast
Mix starter and raisins in mixer bowl, cover and set aside for a few
hours or overnight.
Combine ingredients in the mixer bowl, knead on speed 1 for 1 minute,
the on speed 2 for 4 minutes or until the dough looks right. Remove the
dough, oil the bowl, return the dough and allow it to rise for 1 hour.
Shape and place in a bread pan. Allow to rise for 40 minutes. Bake 35
minutes on 325 convection