Change Management is to take a structured approach to managing changes in Business.
To manage the changes, you need to understand who all the impacted stakeholders groups and key stakeholders are, how and when they're impacted, and you then need to make sure the right actions are in place to manage the impacts.
You also need to ensure the key stakeholders involved in delivering the changes are engaged and supported in their role in the delivery.
There are, of course, lots of different approaches that can be taken depending on what the situation demands and the wider context of how the changes are being delivered.
Managing the changes associated with a new digital product can look very different to managing the changes being introduced as a result of a construction project, but there are common and consistent principles across any change: the need to assess the 'stakeholder landscape' and the impacts; the need for a communication and engagement strategy and the need to measure how successful the changes have been.
Change Management is important because without it, you can't foresee the hurdles and you can't be certain that everyone involved is prepared.
The changes could be managed without using the label 'Change Management', it could be seen as a part of good Project Management or something that those involved are doing naturally. It might seem obvious that you can't make changes without speaking to the right people, agreeing what's required to ensure the changes are successful, etc.
The importance of Change Management is that by following a common set of principles, you are assuring yourself, your organisation and those involved in the wider governance of the business that the appropriate measures have been taken to pre-empt the impacts and manage them with the relevant stakeholders.