In 2009, she extended the State Department benefits to include parters of gay employees.It must be election time, evolving, and picking up every little crumb...
"Hillary Clinton only declared her support for marriage equality in 2013. Before that, she opposed it. "
Since the 90's, she has supported the concept of civil unions that would extend legal rights to same sex couples.(i.e. marriage without the word marriage).
Since marriage & civil unions provided the same legal status, it was really just about terminology.
I don't think Clinton every voted against nor financially supported anti-gay marriage efforts.
In the business world, stockholders certainly don't want their management to openly be against any social issue that would cost them customers.