What is the "gross national saving" for?
The first point is that this is a flow statistic: it is the annual excess of income over consumption, in contrast to level statistics such as gross national debt which are largely accumulations of past flows.

As what gross national saving is for: it funds investment, which affects future income and thus future consumption.  Indeed, one of the tautologies of macroeconomics is that Savings = Investment.

A low savings/GDP figure may indicate excessive consumption or insufficient  income.  It may lead to either inadequate domestic investment or the need to borrow from foreigners to fund domestic investment.

A high savings/GDP figure may indicate insufficient consumption or excessive income.  It may lead to either unproductive domestic investment or the need to lend to foreigners to recycle the funds.

There is no optimal figure for savings. Comparative figures for different countries and global trends over time are published, for example from the World Bank.