Commitment comes from the Latin verb committere meaning to unite, connect, combine, or entrust into someone's charge.  It combines om- ("together") and mittere ("to send or put").  It formed in English from the verb commit plus the suffix -ment (first recorded in the late 1500s).  It evolved from meaning "to entrust or bring together" into legal uses for sending someone to prison, and later transformed into a modern promise, pledge, or dedicated focus.  

According to the Oxford English Dictionary (OED), the noun commitment was first used in English in 1579 and found in the medical and writing work of English physician John Jones to mean putting someone into custody or prison.  The modern sense of making a promise, pledge, or engaging oneself came in 1793 and in 1864 the meaning shifted to mean a general duty or binding obligation.  

A character's commitment drives the narrative forward by anchoring their motivations, raising the stakes of the conflict and making their personal transformation or ultimate success and failure emotionally satisfying for the audience.  Without a clear "buy-in" to their choices, the plot stalls and readers lose interest.  

Driving the Plot and Tension:  
Active choices: Committed characters make hard decisions that push the story forward rather than letting events happen to them passively.  
High stakes: When a character is fully invested in a goal, every obstacle they face carries heavy weight and danger.  
Pacing control: Strong resolve keeps the narrative moving at a steady, engaging speed instead of wandering aimlessly. 

Fostering Emotional Connection:  
Audience empathy: Readers root for a character who cares deeply about what they are trying to achieve. 
Meaningful growth: A deep commitment makes the pain of the struggle and eventual self-actualization feel earned.  
Avoidance of flip-flopping: Firm dedication prevents characters from changing their minds too easily, which keeps relationships and subplots grounded.