Ask HN: What is the value of creating a FOSS as a startup?

Creating a product is a small part of the success of a startup. Sharing the product that you have created with your competitors is giving up the (probably) only advantage that you have over your them.

What possible non-ideological reason would you have to create a FOSS product and how do you not get absolutely murdered by the competitors far superior sales and marketing teams?

Regards