it can be done by a voltmeter and ammeter...........then why we need to convert a galvanometer into "voltmeter" and " ammeter".
I am guessing that you encounter this in a physics class.
The reason why students are asked to do this is more pedagogical in nature, i.e. it is another illustrations of how electric circuits work. But there is also another very important aspect to this exercise. It tells you that these equipments are not "perfect" equipments. when you are measuring the potential difference, for example, you expect that your "meter" itself doesn't affect your measurement. But constructing a voltmeter using a galvanometer and another resistor shows you that there is a range of values for the impedence of the circuit in which your equipment can make an accurate-enough determination. If the impedence of the circuit you are measuring starts to approach the internal resistance of your voltmeter, then your measurement will start to go wrong very quickly.
We need to always be aware if what we measure is what we're supposed to get with minimal influence from our measuring technique. At the very least, we need to know how our measuring technique is affecting what we measure. I'd say that this is one of the first introduction to this aspect of science that a student can get.