Checks have your bank routing info and your account number on it. The thieves just need that info to cause the problems, not the check. Intercept the check, get that info and then send the check on its way without anyone knowing that you now have that info. How much stuff could you buy or get cash transferred with that info via “e-check” before the victim found out and did something to stop it? Then get someone else’s check and rinse and repeat. There isn’t enough punishment meted out these days for that sort of fraud which won’t deter stupid people.
And since it's magnetic ink, they have probably figured out a way to scan the envelope without opening it.
We used to put our phone numbers, driving license numbers and earlier our SS number on the checks.
Never had a problem.
But I haven’t used a check in 15 years.