What is the critical function of reserving starchy pasta water in dishes like Pasta Limone?

Answer

It emulsifies with fat to create a cohesive sauce

The residual starch in the water emulsifies with the fat (butter/oil) to create a smooth, cohesive sauce that coats the pasta rather than pooling at the bottom of the bowl.

What is the critical function of reserving starchy pasta water in dishes like Pasta Limone?
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