In Canada that's the amount where your transaction get automatically submitted to FINTRAC (Financial Transactions and Reports Analysis Centre of Canada). Abnormal smaller transaction patterns and suspicious transaction can also be reported to FINTRAC for analysis.

"FINTRAC analyzes approximately 19 million transactions per year. In 2017, FINTRAC made 2,000 disclosures to police forces." according to Wikipedia.

US banks are generally required to report all cash transactions over $10K. However, they're also supposed to report smaller cash transactions that look suspicious. Like if you deposit $5K cash today, and then another $5K cash tomorrow, then it's likely that the bank will file a Suspicious Activity Report (SAR) with FinCEN.