I'm new to sourdough and I see some recipes use levains and some use straight starter. Why is that? Seems to me that when a levain is used its really just feeding the starter to make it larger like I'd do during a regular starter feeding. If there's a recipe that calls for a levain of 10g starter, 45g flour, and 45g water, why couldn't I just use 100g starter? Thanks!
My answer comes more from microbiology than baking knowledge. The microflora in refrigerated starter have likely reached a sort of equilibrium: they've chewed through much of the food source provided to them, expelled all kinds of waste products, and killed off or outgrown each other to some extent. At this stage, they are quite dormant (if not dead, which many of them are). But when you take them out of the fridge and "dilute" them into your levain, so to speak, you're hitting a kind of reset button. With lots of food and the competition spread out, they can go to town, multiply like crazy and ferment your dough just like you want them to. Doing one or more feed steps builds up a thriving population of active microbes--just what you need for your dough to be properly fermented.
Also worth noting that if you put 100 g of starter straight into your final dough, not only will the microbial activity be very sluggish, the consistency of the dough will be all out of whack too. I suppose this is because the gluten gets way overdeveloped from just sitting around for so long.
the starter. Also many sourdough bakers keep their starter "pure" concentrated and in small amounts (50g) with a different schedule and flour than the formation of the levain. You can easily use your starter replacing the levain in a recipe provided it ready to use and you have enough active peaking starter. Just be careful not to use it all accidentally. Some levains can be quite specific developing the bacteria and yeast in various steps or stages at various temperatures. Use what best works for you.