In normal 555 Astable mode capacitor is discharged through R2(10k) via DISCHARGE pin of 555 which connects to the GND through internal transistor.
When capacitor is discharging, Power source also gets connected to the GND through R1 and uselessly wastes power. to prevent this there is a transistor which turns on only to charge capacitor and turns off when discharge is active.
With Transistor: Min. 2mA, Max 19.9mA
Without Transistor: Min 11mA, Max 20mA
use toggle switch to see the difference.
Note: Do note that when transistor is present it changes frequency due to how transistor functions. make sure to adjust component values to have desired effe
