Watching a russian tv-series about the life of Peter the Great, I didn't quite follow why did he crown Catherine I as Empress ? I looked the Wikipedia article but they don't mention any reason.
There are two types of Empresses. The first type is an Empress Consort, who is the wife of an Emperor. The second is a ruling Empress, basically a female Emperor, who inherited the crown from her father, or in this case, her husband.
Catherine started as Peter's Empress Consort, that is, Empress by marriage. But when Peter crowned Catherine Empress, he had something different in mind, that she would be the heiress apparent, and inherit the throne from him, over say, his children (by either his first wife or Catherine herself). Normally, it's the children, and not the spouse, of a ruler, that inherits the throne, because the children have the blood lines.
By crowning Catherine Empress, Peter gave Catherine the so-called "crown matrimonial," that is the crown through her marriage (more often given by women to their husbands). In so doing, Peter broke with Russian tradition that said only boys could inherit the throne. (Peter was the third, but only surviving, son of his father, so he felt the value of doubling the number of potential inheritors.
By making it possible for women to inherit the throne, Peter gave Russia several Empresses; his wife Catherine, his niece Anna, his daughter Elizabeth, and above all, Catherine the Great who married his grandson, Peter III.