The bar seems to only rain bombs and coins. Rain of coins drops too quickly to be worthwhile. Also, the boost it gives if you collect all the coins is minor. The rain of bombs doesn't give you enough time to defend against it and takes out half your balloons.
Towards the end, press ESC and click on "Upgrades" or "Achievements". Click "Start Playing" and your bomb shield will be reset. Play until you hit N bombs, then press ESCAPE again and repeat the process.