The phrasal verb speak to is widely used idiomatically to convey various senses, including show, demonstrate, express, relate to, address, or speak about. For example, one might say that this post speaks to the meaning of speak to, or that the existence of this idiom speaks to a gap in the language, or that these examples speak to how the phrase is commonly used. The phrase could usually give way to a one-word synonym, but people seem to like using it, especially in speech.
Because the phrase speak to is also widely used in other ways, tracing the exact origin of the newer use is difficult without exhaustive research. For what it’s worth, however, historical Google News and Google Books searches limited to pre-1990 texts uncover no instances (or very few, as we might be overlooking some) of speak to used this way, whereas a substantial number of examples are to be found from the middle 1990s. By the early 2000s, the idiom is ubiquitous.
Why the Duma would give pause on a bill that is supported by more than three-quarters of the population according to some polls … speaks to the delicacy of the gay rights issue in Russia. [Financial Times]
It is an authentic strategic governance action that speaks to the essentials of the board’s role and responsibilities. [Good Governance for Nonprofits, Fredric L. Laughlin and Robert C. Andringa]
But the use of Geronimo’s name speaks to the powerful, if unexamined, hold that the nation’s “Indian wars” continue to have on our popular consciousness. [Los Angeles Times]
It seems important to speak to that struggle, the struggle for meaning and truth in life. [Discovering Darkness in the Light, Dana DeSimon]
The talent and enthusiasm on display in this competition speaks to the country’s enormous human capital. [Wall Street Journal]