For example, mechanical computer printers such as the earliest dot matrix printers may have been able to print 30 characters per second, but when a "carriage return" character was received and the printhead began returning to the left margin, there was a noticeable delay before the printing of the next line could begin.
In its strictest definition, fill characters cause no action to be performed at all; they simply consume time.
In addition, the teleprinter could act on the Line-Feed while the printing mechanism was still moving leftwards, optimizing the use of the data line.
For example the VT05 video terminal sometimes required fill characters when a "scroll up" operation was performed; the process of shuffling data in the shift register memory of the VT05 was slow.
In a similar fashion, communications protocols often require fill characters at points when processing must be performed.