The building provides improvement technologies of the three major unit lines - infantry, cavalry, and artillery while also providing improvements for anti-ship-buildings as well.
Whether players build one or a mix of unit lines, the bonuses provided in the arsenal can give their armies an edge. The "Advanced Arsenal" card available to some civilizations from their Home City adds additional improvements to the arsenal.
"An arsenal is a storehouse for all manner of weapons and ordinance of war. Some arsenals were brick-and-mortar storehouses, guarded and protected. During the lead up to the American Revolutionary War, arsenals could be any stockpile of weapons, often hidden in attics and walls so that British soldiers could not find them. Artillery pieces were even taken apart and buried in fields to hide them, making "arsenals" out of entire towns.
On April 19, 1775, the British marched into Lexington hoping to chase down rebellion leaders and capture an insurgent arsenal. The British regulars outnumbered the revolutionaries 10 to 1 and ordered the gathered militia to stand down so that they could go about disbanding the militia and seizing or destroying any weapons cached in an arsenal."