why should a voltmeter have high resistance?

Voltmeters are put in parallel with the load in order to measure the potential difference between two *different* points.

If they are in parallel with the load, then at the first junction the current will split and flow into both paths (Kirchhoff's Laws). However, if current is flowing through the voltmeter, then it is not all flowing through the load, and the potential difference across the load would change when the voltmeter is added and removed. This is unfavourable. Therefore, the voltmeter must have a very high resistance so that current doesn't flow through it. (In reality, a little bit of current does still flow through, but it is insignificant in most cases.)

Resistance Of Voltmeter

For example,the tension in the voltmeter s poles are 8V and the imtensity of the current that reaches it 0.01mA,according to ohm s law, 8/10^-5 gives 800000 ohm which is indeed a huge resistance.it has to have it cuz electric resistance and electroc tension are proportional (Electric intensity =electric tension/electric resistance)

to stop the flow of current so no voltage drop occur. hence we can measure voltage accuratly

If it has low resistance it could lower the voltage measured.