It's not just about the ethics of investing in these companies. What if you believe (either hypothetically or with supporting evidence) that companies that show good environmental and social citizenship will tend to perform better that those that do not? Then you would consider a fund that is based off of high ESG ratings to be a good investment.
The bottom line is - these funds exist because there is a market for them. Whether the motivation is ethical, financial, or something else, and regardless of the actual impact on the companies that make up the index, there is a market for these funds.