The Church has a hierarchy. Even if instituted acolytes do similar functions to altar servers, they have a degree of authority.
The 1972 Motu Proprio expresses this as:
“He may also, to the extent needed, take care of instructing other faithful who on a temporary basis are appointed to assist the priest or deacon in liturgical celebrations by carrying the missal, cross, candles, etc., or by performing other such duties.”
The 1988 Directory for Sunday Celebrations in the Absence of Priest has:
“30. In the absence of both a priest and a deacon, the pastor is to appoint laypersons, who are to be entrusted with the care of these celebrations, namely, with leading the prayers, with the ministry of the word, and with giving holy communion. Those to be chosen first by the pastor are readers and acolytes who have been duly instituted for the service of the altar are the word of God. If there are no such instituted ministers available, other laypersons, both men and women, may be appointed. …”.
Similarly with blessing ceremonies, from the Book of Blessings:
“18. … d. An acolyte or a reader who by formal institution has this special office in the Church is rightly preferred over another layperson as the minister designated at the discretion of the local Ordinary to impart certain blessings.”