For me? History, mostly. I only own one antique now (well, two, but I'm selling the Nylon 66), a 1942 Finnish M39 on a 1905 czar-marked hex receiver.
From its markings, one can deduce that this rifle was manufactured in Izhevsk, Russia for the Imperial Russian army in 1905. It may have seen some action in World War I, given the rifle shortages the Russian army was facing at the time. It ended up in Finland when the Finns broke away from Russia during the Bolshevik Revolution, was issued to the Finnish Army, and likely saw a great deal of action during the Winter War 1939-1940.
When the Finns got some breathing room in 1942, the rifle was upgraded to M39 status at the Finnish VKT arsenal in Jyvaskyla, where it received a fine Finn barrel, a new stock, and range-compensating Finn sights graduated from 150 meters to 2.0 kilometers. It likely saw additional action against both the Soviets and the Nazis before the war was over.
Possibly during the war, an artistic Finnish soldier delicately carved the initials "E.T." into the rear sight base, where they wouldn't easily be seen by casual inspection. At some point after the war, the rifle was retired and placed into storage as part of the Finnish Army's reserve. During the mid-1990's, Finland allowed many of its older bolt-action rifles to be sold and exported, which is how I came to own it.
The rifle is 106 years old this year (well, except for the barrel and stock, which are only 69), and it can still shoot the center out of a playing card at 100 yards with the right ammunition.
It's not worth a lot from a financial standpoint, but it's a textbook of Russian and Finnish history, and played a part in some of the most important historical events of the twentieth century. That's fascinating to me.
FWIW, I only shoot new-production ammunition in that rifle. Nothing corrosive (or corroded).