The khatib delivers the sermon at the Friday congregational prayer, and the office rests on a precise point: the sermon is not an addition to that prayer but a condition of it.
The sermon as condition
At the Friday midday prayer, which replaces the ordinary noon prayer for the assembled community, the khatib mounts the pulpit and delivers the khutba, the sermon, and in the understanding of the law this sermon is a required part of the Friday observance rather than an optional adjunct, so that without it the special Friday prayer is not fulfilled. The khatib therefore holds an office integral to the central weekly act of Muslim communal worship, and the sermon, which combines praise, exhortation, instruction and prayer for the community, is a fixed element of it.
Who preaches
The role of khatib is often held together with that of the imam who leads the prayer, but the two functions are distinct, and the khatib is appointed or accepted by the mosque and the community rather than ordained, since Sunni Islam has no ordained clergy. The Friday sermon has historically carried great weight beyond the religious, for the mention of the ruler name in it was a mark of sovereignty, and the pulpit has long been a place where the concerns of the community, religious and political, are voiced. The office thus joins a required liturgical function to a public platform, and the khatib is the one who occupies it at the defining assembly of the Muslim week.