In 1990, Mr John N. Collins published his DIAKONIA Re-interpreting the Ancient Sources (OUP). You can probably fiddle around with Google and discover that its conclusions, more than two decades later, have not been disturbed. If you have queries about details in what I am about to write, a reading of Collins will probably answer them; I am not going to summarise him at any greater length than one paragraph.
Collins began by identifying a particular understanding of diakonia which became fashionable in Protestant circles in the middle of the twentieth century; and then infected the Latin Church too. It saw diakonia as meaning self-giving service to the poor and needy. Based on a misreading of Acts 6, it appealed to Christians at a time when ecclesial structures were losing power and prestige. "OK", it cheerfully claimed, "if you've lost your power and status you can still surreptitiously claw it back by asserting the moral high ground of humble service". Collins demonstrated, from examination of profane and sacred Greek usage, that the word diakonia, and its cognates, have a quite different root sense: that of one person's commissioned service to another person.
So the essence of the concept is not the following of Christ who came to 'serve rather than to be served'. The Deacon's basic purpose is not to be washing the feet of the lowest of the low (just as the nature of the Church is not, as we have so frequently been told, to be the Servant Church). Such things may be worthy in themselves ... may, indeed, be the charism of particular holy people. But they are not what diakonia is fundamentally all about.
What is it about? In its essence it is about serving, being commissioned to serve, the Bishop, the Eucharistic celebrant; about serving him in the administration of the Lord's Body and Blood; serving him in the proclamation of the Holy Gospel. Not a philanthropic service but a cultic, liturgical service. In as far as their duties may extend in the direction of philanthropy, it is instructive to observe the role they have in Pseudo-Hippolytus: the deacons are to attend the Bishop and report to him who are sick so that he, if it seem good to him, may visit them. Their ministry is to the Bishop, not to the needy. This role survives almost verbatim in the classical Anglican Ordinal: the deacons are "to search for the sick, poor, and impotent ... to intimate their estates, names ... unto the Curate".