The mainplate is the foundation of the watch movement. It holds together the parts of the watch in an organized and calculated manner. Together with the bridges, a watch is composed of a mainplate in a general view. The bridges are screwed to the mainplate to hold everything sturdily.
No watch can be created without a mainplate. Commonly, mainplates are made of brass but there are some who uses nickel or German silver.
Featured Image Credit: Kroman Watch Works