In the every day normal daily occurrences when we put a date next to our signature, writing a two digit year as 20 is not going to impact you. However, we live in a world where people are out to exploit others. Is it possible that just writing 20 for the year will give those exploiters more opportunities this year for the unsuspecting soul?
How does this affect you? What if you wrote a check to someone dated 11/12/20. The person receives the check, but never cashes it. Then they find the check 11/12/21. Normally, that check is too old to cash. But could the holder of the check add a 21 to the end making the date 11/12/2021 and take it to the bank? The bank certainly would not know you didn't write 2021 on the check if presented for cashing on 11/19/2021. Depending on the amount of the check, this could throw off your account balance and budget. Then what do you do? Try to collect it back from this person?
This brings me to the old saying "better safe then sorry." In a world where hackers are constantly trying to breach stores and obtain account information or hack your email account, do we really need one more thing to worry about? Wouldn't it be easier to make it a habit to just write a four digit date on everything? I don't know about you, but my worry list is long, so a quick, easy change in a habit seems easier in the long run.