These actions should always be there. Always. You may argue that the line

IF @@trancount > 0 ROLLBACK TRANSACTION

is not needed if there no explicit transaction in the procedure, but nothing could be more wrong. Maybe you call a stored procedure which starts a transaction, but which is not able to roll it back because of the limitations of TRY-CATCH. Maybe you or someone else adds an explicit transaction to the procedure two years from now. Will you remember to add the line to roll back then? Don't count on it. I can also hear readers that object if the caller started the transaction we should not roll back.... Yes, we should, and if you want to know why you need to read Parts Two and Three. Always rolling back the transaction in the CATCH handler is a categorical imperative that knows of no exceptions.

convert(date, 'строка', 112)

Explicit conversion from data type int to date is not allowed.

convert(date, cast(20200101 as nvarchar(8)), 112)

Conversion failed when converting date and/or time from character string.