Meir Statman, the researcher and professor of finance at Santa Clara University, wrote that investments offer "three kinds of benefits: utilitarian, expressive, and emotional."
Generating actual investment returns is a utilitarian benefit, and the emotional reward comes when investors feel good or bad about a company and positively (or negatively) about owning part of said company. With sustainable investing, the emotion one feels towards a company and its practices is clearly important to some investors.