If the password you want to change is being managed by some other system, such as Active Directory or SAP, your administrator has created a password policy to ensure that your password meets the requirements of that system.
In many cases, passwords such as these are shared between a number of different apps. Just as importantly, many of the apps give you access to information and actions that are valuable to your employer.
Because your company wants to ensure that no one but you can get to that data, they've configured these password policies to require strong authentication. By providing this extra check when you change your password, IdentityNow can make sure that you are who you say you are before making any changes.