Summary: Understanding the difference between ease of use and context of use is one of the most important UX distinctions. Context of use focuses on where, when, how and why users are performing a task, reaching a goal or using your product or service. Ease of use focuses on helping users get through the interface when they are actually using it. Ease of use is more widely understood, but context of use is far more important to master in your UX strategy.
Function follows context
Ease of use maps directly to usability, a key product goal. It's the most basic idea behind good user experience: Can the user interact easily with the UI and complete their task effortlessly? It is easily understood that ease of use aims to reduce complexity, but rarely do we dig deeper to understand what is going on around the act of use.