Your quotation from the Wealth of Nations is a metaphor that explains why businessmen ,who are uncertainty-ambiguity averse when presented with two options ,where the returns are equal or nearly equal, will choose the option where they know more (domestic investment) over the option where they know less( foreign investment option).Thus,their decision to avoid uncertainty /ambiguity,which has nothing to do with risk assessment, and invest in the domestic market provides benefits (more domestic aggregate investment) to the rest of that society which it was not their intention to provide when they made their decision.It is accomplished as if by an Invisible Hand .There is no Invisible Hand of the Market theory being presented here by Smith,as it follows from the virtue of prudence ,recognized by Smith as the most important virtue in his The Theory of Moral Sentiments,that it is necessary to do the best you can for yourself first before you can help anyone else secondly through the virtue of Benevolence ,which requires a surplus(profit) to be able to apply it.