In gas turbine what is the function of bleed valves?

A bleed valve for bleeding air from a compressor at start-up of a gas turbine engine is self-actuated. The bleed valve moves between open and closed positions based on the pressure within a compressor. 

the bleed valve is driven to an open position at low pressure conditions and to a closed position once the pressure within the compressor chamber becomes higher and closer to operating pressures.

Swe can say it's normal open valve and tend to close when turbine start up .

Bleed Valves (also known as Air Blow off Valves in some other plants) functions to relief the Compressor of excess pressure during start up in order to prevent compressor Surge.

bleed Valves are connected to Compressor as it used to transmitt air from compressors to cool Nozzles and Rotors of turbine

The bleed air valve has special purpose, as the name suggests, it takes a bleed air line from a compressor (hot compressed air) which then can be cooled and utilized as instrument air for a gas turbine. in other words the air processing units APU can be bypassed. There is a special unit named bleed air cooler to cool the bleed air from the compressor.

nice answer mohamed eliwa. In other words to prevent surge for the axial compressor. as some time they named antisurg