It’s really important that you tap in and out when you are travelling with a c2c Smartcard. If you don’t tap in at the start of your journey, your Flexi-Season won’t be activated and you won’t therefore have a valid ticket. This means you could receive a penalty fare.
If you don’t tap out, there’s a chance the wrong product will be used. Plus, you need to tap in and out to be eligible for any credit for travelling off-peak and to receive any Automatic Delay Repay payments you may be due.