What the purpose of the diode rectifier used with a ac analog voltmeters?

The analog voltmeter uses a dial, which changes its position in accordance with applied voltage - however, this voltage must be DC in order for this to work (it's through either thermal or magnetic expansion). So, the AC must first be rectified into DC, before any measurement can take place.

Of course, rectifying AC into DC does increase the voltage (as it must - this action multiplies the voltage value by the square root of two), so before the measurement can occur, this increase is properly sanitized by associated circuitry (for example, a voltage divider could be used).