How to explain percentage performs better than integer?

I have an integer field which means finished mission number. The total mission number is a fixed value. Should I use finished mission rate replace finished mission number as a feature?
The data like this:

userid finished_mission_num total_misssion_num finish_rate
a 5 20 0.25
a 6 20 0.3
a 7 20 0.35

The model I used like this:
The accuracy using “finish_rate” is better than using finished_mission_num.