It is not mandatory, but it would be handy for GDPR compliance. Having a separate record for your IT systems and what processes they are used in will help you stay compliant in practice. According to the GDPR, you must demonstrate that you comply with the regulation according to article 5 (2). A record of all your IT systems and how each is compliant would help you demonstrate compliance with, e.g. articles 25 and 32 on security.