Definitions: Basic science is research conducted without immediate applications in mind. It is driven by the "curiosity [...] to expand [humanity's] knowledge, not to create or invent something." Applied science, on the other hand, is conducted to "solve practical problems of the world, rather than acquire knowledge for knowledge's sake," or to "improve the human condition."
Edit (as per JeffE's suggestion): Let us define 'we' as academics, and 'to whom' as questioners ranging from grad students experienced with research, to sponsors familiar with research, and to the general public (friends, family, etc.) in fields outside of research.
Question: How can we justify funding for basic research? In other words, how can we justify maintaining the status quo for (or even increasing) the funding of research without immediate applications?
Why address this question?
Over the past 10 years internationally, funding in both the public and private sectors for basic research has been decreasing. In 2014, Canada cut funding for basic science in 2014* to "place a new emphasis on applied research."
Likewise, Austrian researchers reported in 2011 of a popular opinion in their country "to [avoid investing] in costly basic research when the results of other countries’ basic research are [...] available without charge."
Finally, a 2014 article in Nature reported of basic funding cuts in favour of "a focus on applied research [...] in Argentina, Canada, Denmark, [France], Italy, Portugal, Russia, Senegal, Serbia, Spain, the United Kingdom and the United States."
Outside of shaping policy, addressing this question can inspire graduate students who may be doubting the usefulness of their research, create an informative resource for undergrads considering a career in basic research**, and provide meaningful answers when asked by the general public (friends, family, journalists, etc.).
Explanatory footnotes:
*More precisely, the CIHR (Canadian Institute of Health and Research) reduced funding as Canada's "federal funding body for biomedical research." Another source by the CAUT (Canadian Association of University Teachers) reported that "federal support for basic research has slowed significantly over the past six years [2007-2013]."
**Interestingly, researchers of a study in The American Biology Teacher "designed an activity in which students take part in a mock grant panel [for disbursing funds for grant applications]. The results indicated a strong tendency toward student funding of applied medical research at the expense of basic research. Exposure to a few examples of important basic research moderates this tendency."