ADConnect is not necessary for deploying a certificate to the phones and deploying an application for Intune standalone management scenario. But it might be used via hybrid scenario. For example, Azure AD Connect will provide the function of Federation, which is an optional part of Azure AD Connect and can be used to configure a hybrid environment using an on-premises AD FS infrastructure. It also provides AD FS management capabilities such as certificate renewal and additional AD FS server deployments.
By the way, for deploying a certificate, the following certificate Connector is necessary for deploying a certificate:
To use the following certificate profile types, you must install the Microsoft Intune Certificate Connector:
o SCEP certification profile
o PKCS certificate profile
To use PKCS imported certificates:
o Install the PFX Certificate Connector for Microsoft Intune.