The Dual Coding Theory, originally proposed by Paivio on 1971, has inspired debate and decades of research.  This theory, described as “one of the most influential theories of cognition this
[20th] century” (Marks, 1997) attempts to explain why mental imagery is so powerful for recall of verbal information. 

Although far more complex than the scope of this blog post, Dual Coding Theory can be simply explained as two separate systems in the brain that work in tandem to lay down memory traces, and which increase the chance that a memory will be created and retrieved.  One system, the verbal memory system, lays down tracks to recall verbal information. When the language is converted to mental imagery (either intentionally and with effort, or spontaneously), the visual memory system is also then engaged.  Thus, rather than having only a single verbal memory trace laid down for the word, the visual imagery laid down an additional pathway, increasing the likelihood of recall of specific meaning. The
chance that the memory for this word will be retained and retrieved are significantly greater when it is stored in two distinct brain locations rather than in
just one location.  

For example,  if the word “prudent” is learned using a definition, but it is paired, either incidentally or intentionally with a visual image of, say, a person making a wise choice, Dual Coding Theory explains why the person is more likely to remember and retrieve the concept.

Although other theories exist, such as Common Coding Theory, The Dual Coding theory has, for decades, been subjected to vigorous criticism and to many
attempts by researchers to refute Paivio’s theory without success.  According to Nigel J.T. Thomas ;,
“Paivio has continued to
develop, elaborate, and defend it, periodically reviewing the relevant
experimental literature.* http://plato.stanford.edu/entries/mental-imagery/theories-memory.html

We know that pairing the language of vocabulary with imagery helps students lay down more effective mental maps for theses words. The likelihood of recall and retrieval is also increased.

*(Paivio, 1971, 1977, 1983a, 1986, 1991a, 1995, 2007; Paivio & Begg, 1981; Sadoski & Paivio, 2001 – for less partisan reviews see Morris & Hampson, 1983; Thomas, 1987; Richardson, 1980, 1999)