What is the purpose of the two 1N4148 diodes used in this circuit?
When pin 3 of the timer is low, the voltage at the base of the transistor is about 1.4 V below Vcc, and therefore the voltage across the 180 \$\Omega\$ resistor is around 0.7 V. So the current through the transistor and LED's is about 3.9 mA, regardless of variations in supply voltage (which I guess is a 9 V battery, and can vary a lot). Hence the LED's should stay at the same brightness for a wide range of supply voltage.

In a few words - it's a current source circuit. (Good one to memorize, it comes up a lot. The other common way is to use a zener instead of the two diodes.)