The when.field.changes will trigger only once to make the two field on form same. User can simply change .t field and print the report. It wont affect the further processing logic.
The when.field.changes is from the standard template of a print session. You can choose to exclude it as well. It is just for avoiding large data selection in case if the .t field is kept zzzzz accidentally by user.
e.g. When the data is huge (records in millions) and user want to print only 10 records, he will put 1 in the from field and if he leaves to field as blank, it will select all the rows if your code doesn't have when.field.changes section.
If the code has it, the to field will be defaulted to "1" as well. User can simply change it to "10" and print the report.
Unless you try to do something beyond what you have already mastered, you will never grow!