The complex event processing paradigm (CEP) has been introduced to detect and react to incoming events in many situations that require near real-time responses, though early detection and reaction to emerging scenarios is desirable and is an active research topic. One approach is to enrich CEP systems in a timely manner from diverse and heterogeneous knowledge sources. CEP enrichment is required to cope with today's business demands such as situational awareness while considering real-time constraints including low latency and high throughput. Existing studies either focus on surveying and evaluating techniques and approaches, or concentrate on specific challenges and requirements relevant to complex event enrichment. To our knowledge, none of these studies focus on motivations behind complex event enrichment and related general challenges. This paper aims to provide an in-depth and formal description on the rationales behind enriching CEP systems with external knowledge, and to investigate in existing literature, the correlation between these motivations and approaches to tackle the problems. In addition, challenges faced by complex event enrichment related to these motivations and approaches are highlighted and discussed.