They've become iconography for vinyl collectors.
RCA developed the 45 to replace 78's. They also marketed a changer/player to go along with it. The big hole is the result of the large mechanism needed at the time for the spindle. Jukeboxes were made to accommodate the big hole. After a while, this became the standard for 45's made in the USA.
Initially, Columbia didn't feel the need for a changer as their LP design could play up to 18 minutes per side. Stereo manufacturers developed one anyway. 45 changer adapters could fit on top of the LP changer spindle.
The iconic 'adapter' enables 45's to play on turntables with a standard sized spindle. I never could see the reason for this as most turntables have spindle adapters. Do you really want to pop this is and out for each play?
I have about 1,300 45's and prefer to use the adapter that came with the turntable. It's much less labor intensive.
Outside of their iconic shape, which used to give me a smile when I saw them, they really have no use. Now it just seems to be an emblem to show others how hip you are - whether you comprehend the full meaning or not. Kind of like Che Guevara.