Understanding the rationale improves complianceEnterprise Architects are often in a role of trying to get compliance with a set of decisions (or rules). If people can't see the rationale for the decisions, and it is not clear that their concerns and issues have even been considered, then they are unlikely to respect them (unless the penalty for not respecting them is severe).
The nature of any decision in a complex domain (from highest level strategies to the lowest level decision on a technical or operational approach) is predicated on a set of goals, factors and beliefs. If we can as much as possible make the basis of the decision clear we can improve the chance of compliance or buy in.
Goals and principlesAny rule must be predicated on achieving some goal (otherwise it would have no purpose). Principles to some extent often reflect a class or kind of goals.
Patterns are decisionsPatterns also reflects sets of design decisions. All design involves trade offs. Explaining the thinking (goals/princioples, factors and beliefs, etc.) will assist in people understanding what patterns fit their circumstance.
The world changesThe other consideration is that we live in a changing world. Goals, facts and beliefs change. If we adopt a religious approach to following decisions or rules without being able to inspect their predicates we are unable to determine if a decision/rule that once made sense remains sensible.
If we record the key goals, factors and belief and how they relate to decisons/rules we can understand the impact of a change e.g. if a goal become more or less important, a factor changes etc.
It is for these reasons that in our solutions relating to managing quality (consistency, best practice, etc. including:reference models, decisions on reference models, patterns, principles etc.) that we seek to get the rationales recorded.