What is the purpose of an arched roof?

Churches require a very large open space that is is either free of columns, or limits columns to the periphery so that the worshipers view to the altar is unobstructed. In modern times, spanning large distances is made relatively easy by the use of steel. However, in those early eras of church construction, the process for making steel had not yet been invented by humans. This meant that the structural materials used in buildings were limited to masonry, wood, and concrete-like mixes. When used as a beam, the physical properties of wood limit the length that it can span to relatively short distances. Thus, the traditional building method known as "post and lintel" (beam and column) was not much of an option if very large column-free spaces were desired.

When humans discovered how to make arches out of stone and bricks, it provided a way to span much greater distances without columns. Thus, the large, column-free churches of the past were made out of series of arches.

Over time, the look and feel of these arched cathedrals became the symbol and standard of a Christian church, and so even in current times when steel beams are available, churches are often made to resemble the arched-construction of the past.

In reality, the arches found in modern day churches are not actually "true arches" that transmit forces by compression only, from the keystone down - but rather steel framed structures that are shaped to mimic the appearance of the arches of the iconic churches of the past.