WIF is a set of .NET classes so it's portable across any .NET applications that wants to outsource authentication and authorization to a STS like ADFS.
It relies on the WS Federation protocol which means that the same claims-enabled application can authenticate with any WS Fed STS e.g. ADFS, Ping identity, OpenAM, Azure Active Directory, Identity Server etc.
All that's required to achieve this is some web. config changes - no code changes.
And it provides a set of methods to enumerate the claim etc.
It also provides the plumbing e.g. decrypting the SAML token, checking the signature and issuer, creating the relevant cookies etc. All of which are necessary from a security PoV.
By all means roil your own - but it won't be portable and you need to think carefully about security.