Can someone tell me what semolina does in pizza baking and Italian breads that make it worth using vs. just using bread flour or AP flour??
Semolina is much coarser than regular flour. In pasta, this is essential, because otherwise you lose a lot of starch in the boiling process. In breads, it's mostly a texture thing as far as I know.
There was a recent thread about semolina and its use in bread or pasta here:
If you read recognized authorities on traditional fresh pasta made in the various regions of Italy (Marcella Hazen, Giuliano Bugialli, Lynne Rosetto Kasper), you will see that typical all-purpose flour is recommended for use in North America. Residents of Italy typically use a flour called "00", or "doppio zero", and it is not very strong at all, because with fresh pasta a tender result (not al dente) is desired.
Semolina is used about exclusively in dried pasta, which is made with a very firm dough that is extruded through industrial-strength dies. There are a few regional fresh pastas (like orecchiete) that contain semolina, but these are unusual. In general, although fresh and dried pastas share the term "pasta", their nature is very different, and that difference isn't just about dryness, but also about the level of chew.
I mean no offense at all toward posters who suggest that semolina be used for homemade pasta. Some Italian Americans might use it for fresh pasta, but very few Italians living in Italy ever do this. Read the Italian authors listed above and you will see that is so.
Semolina does have traditional uses in some southern Italian breads, such as varieties from Apulia and Sicily. It is sometimes preferred by artisanal bakers for use in dusting a baker's peel before sliding loaves in an oven (perhaps because it is ground from wheat) instead of cornmeal.
I don't doubt that there is someone somewhere in Italy using semolina for pizza dough, but I would venture that this is also not the norm. Certainly in Napoli it is not.
Yummy taste, lovely texture and color. A small amt up to half does wonders and changes the bread to something else.