Why resistance should be increased for Photoresistor in night for Street Lamps?
A photoresistor or 'light-dependent resistor' (LDR) is a resistor whose electrical resistance decreases at high light intensity.

In the dark, they have a high electrical resistance of a few mega-ohms, whereas during the day, the resistance drops to a few hundred ohms.

This is a physical fact about LDRs.

Electrical conductivity is the inverse of resistivity.

During the day, the current through the LDR is high, due to its low resistance. Conversely, at night, the current through the LDR is relatively low. This fact can be used to 'drive' a relay coil, for example, so that the relay is energised during the day. By connecting 230VAC to the light circuit via a normally-closed contact from the relay, the street lamp(s) can be made to turn OFF during the day and ON during the night.

Here's a simplified schematic:



In practice, this would be done with solid-state electronics such as transistors, so as to improve efficiency, reliability and cost.