This is a very nice text from Aṅguttara Nikāya. Thank you for posting it and very good question. I like the various answers, and will add my own. When we are mindful of the body, who is being mindful? To whom does the object of perception come? I believe by investigating this we can find the deathless, and I suggest this is what mindfulness of body, feelings, etc. is about.
The sense stimuli and the sense base giving rise to consciousness leads to a union of the three. This causes contact. In turn, from contact arises vedana, sanna (perception/identification) and intention (+ other mental fabrications).
The Buddha was dissatisfied as long as there was even the modicum of disturbance hence why he left his early teachers as their teaching lead only to the immaterial attainments, as refined as they were, didn’t lead to nibbana or full cessation of phenomena without any disturbance or suffering related to impermanence (aka ‘sankhara dukkha’).