It is neither a contract nor a tort remedy, but a right based on principles of fairness. Contribution and subrogation overlap, but there are instances where one may be applicable and not the other. Assignments are usually based on a contractual agreement between two parties. As such, an assignment differs from a subrogation or contribution in that an assignment is generally not based on the same legal principles of fairness. An insurance company may seek a voluntary assignment of rights from an insured or another person if the insurer thinks its position in court as an assignee will be stronger than as a subrogee.
Subrogation has lots of traps for insurance companies.
Judges tend to think that insurance companies are supposed to pay claims and be done, and there is some reluctance to let them recover their payments from others. The result is a minefield for insurance companies. In other words, if an insurer was not liable for payments, then it cannot recover payments made. This can lead to court battles as to whether an insurer really had an obligation to make payments. In these days of bad faith concerns, an insurer may sometimes make questionable payments to avoid getting sued for bad faith, and that may jeopardize the chance of recovery.