Your questions are addressed in the excellent chapter on the Eucharist of the Catechism of the Council of Trent. For example, it says that as the faithful
deem it necessary to afford daily nutriment to the body, they should also feel solicitous to feed and nourish the soul every day with this heavenly food. It is clear that the soul stands not less in need of spiritual, than the body of corporal food. Here it will be found most useful to recall the inestimable and divine advantages which, as we have already shown, flow from sacramental Communion. It will be well also to refer to the manna, which was a figure (of this Sacrament), and which refreshed the bodily powers every day.
Humans are comprised of body and soul. Jesus knew this when he quoted Deuteronomy 8:3 in Matthew 4:4 and Luke 4:4:
…not in bread alone doth man live, but in every word that proceedeth from the mouth of God.
He does not say we live solely off the "word that proceedeth from the mouth of God" (like the angels), nor in "in bread alone" (like the brute animals), but in both.
St. Thomas Aquinas, addressing the question of "Whether the sacramental species
[i.e., what appears as bread and wine in a consecrated Host] can nourish?
" (Summa Theologica III q. 77 a. 6
…food nourishes by being converted into the substance of the individual nourished. Now it has been stated (a. 5) that the sacramental species can be converted into a substance generated from them. And they can be converted into the human body for the same reason as they can into ashes or worms. Consequently, it is evident that they nourish.
But the senses witness to the untruth of what some maintain; viz. that the species do not nourish as though they were changed into the human body, but merely refresh and hearten by acting upon the senses (as a man is heartened by the odor of meat, and intoxicated by the fumes of wine). Because such refreshment does not suffice long for a man, whose body needs repair owing to constant waste: and yet a man could be supported for long if he were to take hosts and consecrated wine in great quantity.
In like manner the statement advanced by others cannot stand, who hold that the sacramental species nourish owing to the remaining substantial form of the bread and wine: both because the form does not remain, as stated above (q. 75 a. 6): and because to nourish is the act not of a form but rather of matter, which takes the form of the one nourished, while the form of the nourishment passes away…
The sacramental species nourish the communicant's body not as bread or wine, because there is no bread or wine in a consecrated Host and consecrated Precious Blood. After consecration, the substances of the bread and wine are replaced with that of the Body, Blood, Soul, and Divinity of Christ Himself (transubstantiation).
"Christ's body is not changed into man's body, but nourishes his soul." (ibid. ad 1).
He said: "For my flesh is meat indeed: and my blood is drink indeed." (John 6:56).
Also, several saints and blesseds have been known to subsist solely off the Eucharist (cf. ch. 36, "Eucharistic Fasts," of Eucharistic Miracles: And Eucharistic Phenomena in the Lives of the Saints).