As explained in the article, if you don't have 25k€ of capital, then you open an Einzelunternehmen (sole proprietorship), not a GmbH (LLC). It's a much simpler process.
If you are selling Hot Dogs, nobody will bat an eye if it’s a UG, but if you apply for a big software contract, people might be wary.
It seems to me that a sole proprietorship would do more to protect against counterparty risk, i.e. more of the person's assets would be available to satisfy the debt.
My guess is that counterparties prefer a limited liability partner to arguably insulate themselves from employment liabilities.