Legacy systems may be critical for the successful operation of a business for two basic reasons
1. They may be an intrinsic part of one or more processes which are fundamental to the operation of a business. For example, a university has a student admissions process and systems that support this are critical. They must be maintained.
2. They may incorporate organizational and business knowledge which is simply not documented elsewhere. For example, exceptions on student admissions may simply have been coded directly into the system with no paper record of these. Without this system, the organization loses valuable knowledge.