MIME is an Internet-standard method for passing data on the Internet. No longer are you limited to typing text into an e-mail program. An attachment is just a collection of material that will be placed into your e-mail message using a particular format. The MIME format is implemented in many e-mail programs from many different vendors on many different kinds of computers. It is the Internet standard.
Internet standards are important.
An RFC is a request for comments, meaning a document prepared by an Internet working group and then adopted by Internet standards setting bodies. All RFCs are public and in the public domain. Any software author can implement formats and procedures they prescribe. Programs that implement RFCs correctly can then interoperate with other programs that do the same. The Internet is built on such standards.