The word "denominations" suggests that the items under considerations are all simply variations on the same thing. The term "confessions" is intentionally used to relate the idea that those who align themselves under the name "Christian" are not necessarily all part of the same sliding scale, the same spectrum, ranging from conservative to liberal. What follows come from the Foreward to the translation of Werner Elert's Elert's Morphologie des Luthertums, The Structure of Lutheranism (Concordia Publishing House: St. Louis, MO).
One of the earliest English names for a “Lutheran” was “Confessionalist.” [cf. OED II, 1933, p. 802]
Because the Lutheran Church defined itself in a series of confessions but never adopted an official liturgy or a uniform polity, Lutheran theologians have often supposed that the key to understanding any section of Christendom is its confession or symbol. Thus has arisen the branch of theology called Symbolik, or more recently Konfessionskunde.
In fact, the German word for “denominations” is Konfessionen; but at least since the eighteenth century American English has been calling the Konfessionen “denominations.” For even though academic theologians may wish that the denominations were confessions and expressed their genius in the form of a statement of faith, the mute realities of history make it clear that “in Great Britain and America … the chief differences between the religious denominations are not doctrinal but institutional. If therefore any one wishes to make a comparative study of the consensus and dissensus of British and American Christianity, he must pay more attention to religious institutions than to doctrines of Faith and Morals.”
Therefore American denominationalism as a religious and historical phenomenon5 has been the despair of scholars in the field of “comparative symbolics,” who prefer the neater and more precise interpretations that come from a comparison of confessions and creeds.