Observation is not just looking. It is noticing what matters early enough to make a safe decision. A mirror check should tell you whether someone is close behind, overtaking, slowing, or likely to be affected by your action.
Blind-spot checks matter whenever another road user could be beside you and outside mirror view. This is especially important before moving off, changing position, or joining traffic.
The best learners build a rhythm: check, understand, signal if needed, position, and act only when it is safe.