From the 4th century onwards the law of the Church: Canon law was developed and used. Canon law was applied by ecclesiastical courts.
Canon law is largely comprised of Roman law, it interlinks very closely.
Why is Canon law important?
It has expended the jurisdiction of spiritual courts (officialates): criminal law, family law, private law (contract law > breaking a promise is a sin) etc. Besides this, the Romano-canonical procedure was applied/developed in ecclesiastical courts. This was due to the fact hat eventually no-one wanted to use to old, expensive and strict secular courts anymore > Romano-Canonical-procedure became the everyday-procedure.
As said before on of the reasons that the Romano-Canonical procedure gained popularity was because it was less strict and cheaper. HOWEVER, it also provided new methods to prove your case e.g. medieval oath-helpers (that unlike witnesses, only made a statement about the credibility of the party taking the principle oath) & it was WRITTEN, inquisitorial (instead of acquisitorial). Last but not least, it helped the reception of substantive Roman law.