Difference in Starters

Hi Nora, I had a similar question about hydration of a motherdough - a thick starter fermented in advance of final mix. Here, its called by a generic name of preferment.

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wetter preferment= more protease action and lactic acidity.
drier preferment= lesser protease action, higher acetic acid. 

You would add a wetter type poolish preferment to a drier dough to make an open crumb with an extensible dough like for a baguette.

You would add a drier biga to a wetter dough to obtain strength, like for a ciabatta. 

You would use a sponge for sweet dough, regular sandwich loaves, etc as it is in between, imparts some strength and flavor.

You can store a dry preferment longer in the refrigerator. You can use a wetter preferment more quickly. 

There are lots of reasons for different preferments, it is like mixing colors on a palette. You can have cold prefements, warm preferments, whole grain, wet , dry , in the middle, etc. 

Acidity is necessary for Rye breads, a preferment brings acidity to the dough.

When you preferment whole grains, you predigest parts of the grain that help make it easier to digest and less toxic for human consumption. For instance it softens the bran layers and breaks down the phytates.

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While this is for a preferment - the wet vs dry section at the beginning would seem to be applicable to starter cultures - not just motherdough.

'Hope it helps some. I have a post now w/ other questions about starters also.