This word Isvara should not be associated with any devotional systems, or the God of the religions. Patanjali s Isvara is quite a different thing altogether. It is a pure deus ex machina , a contrivance that has been made necessary to explain the relationship between one thing and another. Patanjali had his own arguments for positing the existence of Isvara . It was felt that there cannot be only two parties in a case.
People do good, people do bad.
There is a reaction set up to every action, good or bad. A client cannot be a judiciary. There is a third element necessary, a judge in a court. This judge was introduced by Patanjali, and he called this judge Isvara . If Purusha can be independent of Purusha and vice versa, one body of matter can also be independent of another body.