Line composers are used in programmes that are intended for printing on A4, Legal or Letter:
If the end of a line is reached and you continue typing, the now full line is finalised - and remains so if you do not change anything in it (or in an earlier line of the same paragraph).
Paragraph composers always look at entire paragraphs and adjust the lines they contain as evenly as possible:
Less saccade disruption, better reading rhythm, so more intense immersion. 😊