Sutra:
21. prapaththiyilE izhinthavarkaLukku SvarUpagnyAnaththai piRappiththuk kAlakshEbaththukkum bOgaththukkum hEthuvAy irukkum.

Meaning:
For those who are interested in nothing but prapatti to the Lord, the Thirumanthra will make them understand the nature of their soul which is that it is enslaved to Him, it will facilitate the passage of time by its meanings and utterance, and by its own nature will be very sweet to them.

Sri PBA Swami's Sarartha Deepikai:
The yogas talked about before (karma, gnyana and bhakti) are not easily practised. Prapatti on the other hand is easy and for those who follow that path, the Thirumantra makes them understand their true nature of being enslaved to none but the Lord; it also is useful in passing their time by its utterance as well as learning its inner meanings; it is also by it's very nature very sweet to study and recite - as said by Thirumangai Azhvar in the pasuram "enakkenRum thEnum pAlum amuthumAya thirumAl nAmam, nAnum sonnEn namarumuraimin namO nArAyaNamE". That is, it is most pleasing to experience, just as the Lord is ("enakkuth thEnE pAlE kannalE amuthE thirumAlirunsOlaik kOnE").