Why is Azure Active Directory Used ?

Azure Active Directory is a cloud based Identity and Access Management services which can provide access to the internal and external resources

Active Directory, whether on premise or in the cloud, or both, provides Role-based Access control in conjunction with Identity and Access Management. It is based upon an LDAP/Edir protocol and enforces access only to those who are authenticated and authorized to access resources, whether they be in the cloud or on premise

Azure active directory is a microsoft’s service which helps to signin and access resources such as Azure portal, microsoft 365 and other applications. To know more refer this link,
https://docs.microsoft.com/en-us/azure/active-directory/fundamentals/active-directory-whatis