Why we want to convert a voltmeter to an ammeter or vice versa?
Moving coil instruments are pure current measurement devices: The coil is inside a magnetic field, so a current will cause a momentum on it. As the coil can move, it follows this momentum, but there is also a spring generating a counter-momentum. So, the coil only turns by a certain angle, which depends on the current. A needle attached to the coil allows to read the current corresponding to that angle.

If you want to measure voltage, you can put a resistor in series with the instrument. The voltage causes a certain current, which is then measured.



On the other side, the measurement device inside a digital multimeter (DMM) is a pure voltage measurement device. 

If you want to measure currents, you put a resistor in the line and measure the voltage drop across it.



The point about ranges is that a device usually has a certain range. You need a circuit to map the input range and type (voltage, current) to the range and type accepted by the measurement device.



Finally, your question is:


  Why in laboratory experiments do we want to convert a voltmeter to an ammeter and vice versa? What is our main aim here that is for what we are doing it?


First, I hope you know the difference between voltage and current, and that you usually can not get the one from the other. You you definitely want to be able to measure both.

Usually, you don't have a Voltmeter or Ammeter, but a device where you can select the input type and range. So usually no need to convert it yourself! 

Sometimes, you can not measure the type you desire. For example, there is no way to put your Ammeter into a track onto a PCB. In this case, you may be able to measure the voltage drop on a resistor on your PCB and convert it to current yourself.

I remember we had a pure Ammeter, which can measure some picoAmpere with a high precision. 
If you want to measure a few microVolt, a standard voltmeter may not be able to measure it precisely. But this Ammeter would be, if you convert it to a Voltmeter by a resistor.

However, you may have converted your device yourself during a practical course to learn how it works. And each device has some disadvantages (Voltmeters should not draw current, but they do. Ammeters should not drop voltage, but they do), and you may have investigated this during a course.