collars were a common men's fashion accessory
in the last half of the 19th century. Neck ties had come into fashion and they were traditionally worn with high stiff collar
s. Collars absorb a lot of sweat and tend to get dirtier than the rest of the shirt ("ring around the collar"). They need a lot of washing where as the rest of the shirt doesn't. So attached collars tend to wear out long before the rest of the shirt.
Now that's all well and good when you can run down to The Gap
and buy a new shirt for $19.99 but in the19th century
not only were there no Gaps (indeed) but a nice linen
shirt was friggen expensive. To keep from having to throw out a perfectly good shirt with a worn collar, removable collars came into fashion. Initially removable collars were linen but tossing out linen collars proved to be expensive. Clothing makers soon developed much cheaper paper
collars. Unfortunately those didn't last long and they weren't very comfortable or flexible. In 1870, manufacturers came out with removable celluloid collars which were cost effective, lasted five times longer than paper collars, and proved to be much more flexible and comfortable than the paper kind.