Originally, customers used ADFS for Office 365 (well, technically ADFS trusts Azure AD, not Office 365, Office 365 is more a suite of products) to get Single-Sign-On (SSO) with their on-premises domain-joined machines.
Now we can have SSO without deploying ADFS (Azure AD Connect Seamless-SSO, Azure AD joined machines, Windows 10 and primary refresh tokens etc.). So deploying ADFS for Azure AD integration is not required at all. They might be other reasons why customers wants to use ADFS (such as a custom MFA provider). But that's rather rare now.
ADFS has an infrastrucutre cost. There are servers, load-balancers, certificates to manage... So if that's not required to get SSO to access "Office 365", why bother...