How important is long term follow-up care?

Very important in. Varying conditions. Indications for additional treatment and for types of rehabilitation are made by extended follow-up and results can definitely improve in many conditions where there is long-term follow-up.
Depends. That is a vague question. The answer depends on what is being followed up. Simple problems may need no follow-up. Other problems may need life-log follow-up care.
Very important. This is somewhat of a broad question. Chronic conditions like heart disease or diabetes require regular follow up. It ensures that the condition is stable and not progressing, causing problems that might easily be fixed. But even healthy people should visit their doctor regularly to moniter for early evidence of disease and to obtain advise on health maintenance. Regular care is very important.