This is done if there is any suggestion of an abnormal rhythm or heart block occurring intermittently. This might be because of an abnormality on the resting ECG (e.g. extra beats or a minor degree of heart block) or a symptom like a dizzy spell or blackout or severe palpitation (a sense of an abnormal rhythm, either irregular or fast ).
The type of recording will depend on the frequency of the symptom and might be a 24hr or a 72hr tape, or a recorder to activate in the event of a symptom, or a recording device implanted under the skin for an extended period.