Why did CDSL send an email with the transaction details?

Securities are held and maintained by depositories on behalf of investors. CDSL sends an email and SMS containing details of the debits and credits on T+1 day when transactions are settled

Zerodha also sends a monthly holdings statement with the details of all debits and credits in the demat for the given month. To learn more, see How to download transaction cum holding (SOT and SOH) statement?