Observations are not always accurate because real world observations some times miss stuff. A critical event may occur when observations are not being made. A critical event may occur outside the range of the observer’s sight.
Because of this observations can subjective since it is too easy for an observer to see what he wants to see because the observer is looking for what he wants to see So patterns resembling what the observer wants to see are more likely noticed.
It is too easy for an observer to not see what he does not want to see because the observer is not looking for what he does not want to see and patterns not resembling what the observer wants to see can be over looked. Knowing of such problems helps avoid them.
Observations are an important scientific tool but it does have its limitations and pitfalls. Being aware of those limitations helps one to avoid them.