Local value is a vague concept describing that motivates us to prefer to buy local. We see it as a price premium that people will pay to buy a local good over an import. See import substitution for more on exploiting this as a strategy. However, for some people local value is near zero.
When it exists, it is probably a hard-to-untangle combination of moral values shared by people who live nearby, long-term price values (knowing that we gain access to a steadier cheaper supply of something if we buy from those who live and work nearby), and ecological value (preferring not to buy things that use too much transport energy or come from so far away that we can't investigate how they were made).
It is associated to the source's physically closeness to us and their ability to protect what we value emotionally, e.g. local food sources might be more trustworthy to us because we know that we are not exposing our children to more harm than they would normally encounter in their environment (note that this applies in both very pure and very polluted environments). It is also important to preserve local economy and rare local food stuffs, e.g. Slow Food movement.