Why would you "throw" a party?

To supplement @njboot's answer.

I offer the consideration that to throw a party was originally an informal quasi spontaneous do. It suggested the idea that the host(s) had just grabbed some cheap booze, snacks and there was going to be lots of music and people.

Consider also the expression: to throw on something, which means to wear whatever happens to be on hand. The "throw" idiom is NOT implying that you hurl objects at someone or something, but that you throw a party on a date, for somebody or at a specific time/place.

• For example, families of boys named Nicholas throw a party on December 6— Saint Nicholas Day
• You've simply got to let me throw a party for you, Louise," Sally pleaded. "But why?" Louise asked. "So we can announce your engagement publicly."
• She can throw a party at a moment's notice and often does.

Ngram chart confirms that the idiom started life in the early 1920s

The earliest reference for "throw a party for" is dated 1921 in The Delta of Sigma Nu Fraternity, Volume 38

According to the OED, the term dates back to 1922, when Sinclair Lewis used it in Babbitt. I tracked down the passage:

"They and their set worked capably all the week, and all week looked forward to Saturday night, when they would, as they expressed it, “throw a party;” and the thrown party grew noisier and noisier up to Sunday dawn, and usually included an extremely rapid motor expedition to nowhere in particular."

From what I can gather, it seems Lewis used it with the idea of "to toss" in mind, which principally means:

1) [ with obj. ] throw (something) somewhere lightly, easily, or casually

Additionally, it's used as a colloquial term within the passage itself: "as they expressed it, 'throw a party.'"

As defined now, the verb "to throw" includes:

6) [ with obj. ] give or hold (a party).

(Definitions from New Oxford American Dictionary)

The OED has "throw a party" from 1922 but I found several antedatings.

It shows up in US university and fraternity magazines in 1919, 1920, 1922. There's also a 1917 in a story in Good Housekeeping.

The earliest I found is from The Scroll of Phi Delta Theta (Volume XL - May 1916 - Number 5 - page 457):

CALIFORNIA ALPHA
University of California
(No letter received)

CALIFORNIA BETA
Leland Stanford University
...
California Beta "threw a party" at Menlo Country Club on April 28 which was one of the best for years. That is what all the reporters say and we fully agree. Some of the brothers from California Alpha motored down and from what they said they had a fine time.