Verse 2.21: O Pārtha, how can a person who knows that the soul is indestructible, eternal, unborn and immutable kill anyone or cause anyone to kill?
The principle of utility is being highlighted in today's verse. If someone fully understands something, it is only then that they can utilize it to it's full extent.
In this case, when one fully understands that the soul is indestructible, eternal, unborn, and immutable, it is then that spiritual life can begin. It's as though everything falls into place for the bhakti practitioner and all the questions and confusions that may arise due to misidentification with the body just washes away.
Everything has a purpose and whether an aspiring spiritualist or not, we all experience it in our lives. Why else do people feel like they are "meant to do" something? The bhakti texts speak about this in fact. They state that one contributes effectively to society when one works according to their nature and inclinations. The sad truth remains, however, that nowadays very few are encouraged to actually contribute to the world by honing and utilizing their talents and skills. Instead, security and money are the goal. So even if one is operating in the consciousness that "I am this body", they cannot be satisfied by trying to do something that we are not inclined towards. It's like fitting a square peg in a round hole.
Interesting, isn't it? This is the depth of the bhakti texts. Although ultimately they are there to help us develop our dormant love for God, they simultaneously provides us with information on how to function effectively in the material world. They tell us how to usefully engage our given body and mind. If you think about it, it makes perfects sense. Although we are not of this world, we are currently residing here. If we are not guided as to how to operate in the material world properly, then we will not only be unsuccessful materially but also spiritually!
For those who want to take it a step further, the bhakti texts then introduce the fundamental truth that we are not this body but actually the spirit soul. That knowledge is one of the keys to advancement in spiritual life. From there, one can now utilize the body in a way that serves the soul's actual purpose. So one has choices. One can learn how to utilize the body effectively for the sake of the body (which is temporary and subject to birth, disease, old age and death) or one can learn how to utilize it for the sake of the soul (which is eternal).
So in this way the body can be utilized either to gratify the senses or in devotion. So what does that mean? This is where bhakti is so practical. The practice of bhakti is not about negation but one of proper engagement. Those very senses which we use as a means to satisfy ourselves can instead be used in service of God. By our hands we can offer items in gratitude, by our lips and tongue we can glorify him, by our legs we can visit places of pilgrimage and so on and so forth.
It's our choice. We can continue in our vain attempts to satisfy the body through the senses or use our body in the service of satisfying God, which will naturally make us happy. This goes to show, that despite the fact the body is temporary, it is so useful! It is the vehicle by which we can show our devotion to Krsna.