What are the benefits for a country of keeping its currency exchange rate low?

To compete well in Exporting with other exporting nations.

Good percentage of those nations are keeping a low fixed exchange rate.

Countries prefer a fixed exchange rate regime for the purposes of export and trade. By controlling its domestic currency a country can – and will more often than not – keep its exchange rate low. This helps to support the competitiveness of its goods as they are sold abroad. FX fluctuations in other currencies can also have additional  arbitration advantage if properly dealt with.