One function of footnotes is to provide support for statements in the text. Providing authority to support assertions is essential to accurate and persuasive legal writing. Cite form is, of course, governed entirely by the Blue Book. This question is answered in relation to the nature of the case, the specific area of law, and the type of assertion being made. Secondary authorities may be cited as added support, but the preference is for primary authority.
Primary authority is mandatory for support of statements of substantive law.
Primary authorities are cases and statutes. Secondary authorities include such sources as law reviews, treatises, A.L.R.'s, etc. The second function of a footnote is to expand upon the idea stated in the text by giving additional information which, though of interest to some readers, is not generic to the central argument. In expanding an idea, the author can provide historical background, a break down of the court's opinion, treatises, comments on point, the text of the relevant statute, a dissenting position relevant to the idea in the text, or an interesting yet tangential problem. If the author included these types of information in the text, it could direct attention away from the writer's main purposes or bog the reader down in a myriad of facts.