Syncing your calendar with CatchApp has two main advantages.
It enables you to view your existing calendar c commitments inside of CatchApp. This is really useful when selecting free timeslots to offer to someone, so that you don't double-book.
On iPhone, whenever a CatchApp meeting is confirmed, the meeting is automatically pushed to your Apple or Google Calendar so that it automatically appears there. Similarly, if the meeting is cancelled, it is automatically removed from your Apple or Google Calendar.