For the same reason you learn about other periods in history: to learn how things came about and through that, to understand the present situation. Without learning about the 20th century birth of the Middle East nations, the Gulf wars - and most of all, their causes and aftermath - how could you fully understand the present war in Syria, why IS is fighting there, why Saudi Arabia and Iran are now at loggerheads and why the US is now turning itself in 'Fort America" and behaving as if constantly under siege?
because it just is
it is not capitalized because the twentieth century is not cool
On the Twentieth Century was created in 1978.
No, it is mid to late twentieth century
The Twentieth Century Approaches was created in 1986.
The ISBN of The Unfinished Twentieth Century is 9781859847800.
The Myth of the Twentieth Century was created in 1930.
Twentieth Century - typeface - was created in 1937.
The Hoax of the Twentieth Century was created in 1976.
Paris in the Twentieth Century was created in 1994.
Twentieth Century Eightball was created in 2002.
The duration of The Twentieth Century Approaches is 1.7 hours.