Takmil adds a qualifying or supplementary element that makes a meaning more admirable or effective, although the bare statement would already make sense.
وَيُطْعِمُونَ الطَّعَامَ عَلَى حُبِّهِ
They give food to others despite their own love for it. (Al-Insan 76:8)
Giving food already denotes generosity. «عَلَى حُبِّهِ» adds that the food is given despite attachment to it, showing sacrifice and a higher degree of generosity.
Difference from ihtiras: ihtiras blocks a mistaken inference, while takmil enriches the merit already expressed.