The first collect after the Suffrages has always been the Collect of the Day. The 1979 Prayer Book made this optional, but ours restores its requirement. The use of the Collect of the Day outside of the Communion liturgy seems to have originated with Archbishop Cranmer himself. This is, in a sense, a Reformation answer to the tradition of Daily Mass: rather than expecting the people to watch the priest celebrate the Sacrifice of the Altar throughout the week, the primary concern was that the people would come and hear the Word of God read throughout the week. The inclusion of the Collect of the Day was therefore an acknowledge of prior tradition both by acknowledging the liturgical calendar in the daily services and by bringing to peoples’ minds the previous Sunday’s prayers and lessons miniaturized in its Collect.
The purpose of using the Collect of the Day here in the Daily Office is either to bring to mind the Communion service on the previous Sunday or the present holy day. The majority of the Office is quite static, unmoved by liturgical season or other occasion; this Collect is its primary link to the sacramental life of the Church.
Although the Collect of the Day no longer relates directly to the readings in the modern Communion lectionary, its repetition from the previous Sunday or present holy day still serves as a tangible link between the Daily Office and the Holy Communion.