Why do Catholics have reservation of the Eucharist? The tradition is very old and in the beginning provided a practical way for Communion to be reserved until given to the dying:
5. The primary and original reason for reservation of the eucharist outside Mass is the administration of viaticum. The secondary ends are the giving of communion and the adoration of our Lord Jesus Christ present in the sacrament. The reservation of the sacrament for the sick led to the praiseworthy practice of adoring this heavenly food that is reserved in churches. This cult of adoration has a sound and firm foundation, especially since faith in the real presence of the Lord has as its natural consequence the outward, public manifestation of that belief.[Eucharisticum Mysterium 49]
Adoration of the Blessed Sacrament is a later development in Catholic practice. Likely it was observed locally before being officially endorsed by the hierarchy.
What is real presence? Section 6 gives one of the best official explanations, noting that the Church sees these manifestations of Christ as a progressive experience, rather than a hierarchical one:
6. In the celebration of Mass the chief ways in which Christ is present in his Church emerge clearly one after the other. First, he is present in the very assembly of the faithful, gathered together in his name; next, he is present in his word, with the reading and explanation of Scripture in the church; also in the person of the minister; finally, and above all, in the eucharistic elements. In a way that is completely unique, the whole and entire Christ, God and man, is substantially and permanently present in the sacrament. This presence of Christ under the appearance of bread and wine “is called real, not to exclude the other kinds of presence as though they were not real, but because it is real par excellence.”[Mysterium fidei 39]
Pope Paul’s is quoted here. He draws from a larger thought in his encyclical that includes the Tridentine teaching of the Eucharistic Real Presence being “substantial,” or perhaps, deriving from the “accident” of substance or matter.
Modern liturgists would adhere to a certain ritual reverence for the Scriptures and the assembly, but we would not accept a “substantial” presence in the physicality of the Lectionary, Gospel Book, or the physical gathering of the faithful.
No Real Presence on the altar as Mass begins:
Consequently, on the grounds of the sign value, it is more in keeping with the nature of the celebration that, through reservation of the sacrament in the tabernacle, Christ not be present eucharistically from the beginning on the altar where Mass is celebrated. That presence is the effect of the consecration and should appear as such. [Eucharisticum Mysterium 55]
A reminder that hosts in the tabernacle are for the sick and for others who must receive outside of Mass. No mention is made of having a “convenient” supply for distribution at Mass:
7. The consecrated hosts are to be frequently renewed and reserved in a ciborium or other vessel, in a number sufficient for the communion of the sick and of others outside Mass.[ GIRM 285, 292]
With some difficulty in some locations, security issues can impede #8 somewhat:
8. Pastors should see that churches and public oratories where, in conformity with the law, the holy eucharist is reserved, are open every day for at least several hours, at a convenient time, so that the faithful may easily pray in the presence of the blessed sacrament.[Eucharisticum Mysterium 51]