Why A voltmeter is connected in parallel with the point being measured, so it forms a parallel path for current. If its input impedance is low, it draws appreciable current and changes the voltage it is trying to read, especially in high-impedance circuits. Making the input impedance high (typically 10 megohms or more in a modern DMM) keeps that drawn current tiny so the circuit behaves almost as if the meter were not there.
Watch out High impedance does not extend the voltage range or improve resolution; those come from the range divider network and the number of digits in the analog-to-digital converter. Safe measurement of high voltages depends on insulation and probe rating, not on input impedance.
Voltmeter goes in parallel, so it should sip, not gulp: high Z means low loading.