The Opto coupler diode is a "slow" diode. The zener diode has a fast turn on time. The spikes from an ignition coil can be VERY fast rise time to high voltages. It can zap the opto coupler. (It has happened to me in similar applications!!) The zener diode solves this problem. And it is not very expensive.

But hey, if it works for you without it then go for it. I just don't recommend it. Vehicle ignition noise is quite impressive. My point in presenting the circuit (new readers need to reference earlier thresd) was to show however that there are relatively easy ways to interface and deal with the direct coil interface.

And aside from the RPM that Aman Khalid was interested in obtaining there are other engine monitoring functions that direct coil monitoring can permit. Other sensors from the more modern cars may not afford the same functionality that this approach gives.
Mike Karas