Cast carriers, in one form or another, have been around for along time. In the days of gut casts , they came in the form round tin, containing a moist fabric pad, essential to keep the cast damp and in good condition. Modern nylon cast are stored dry of course, and cast carriers have been made of a range of materials such as card or plastic, like the one illustrated above, - you can even make your own from scrap card.
Carriers like this are popular in the competition circuit, where it's important to be able to change a team of flies quickly and without fuss - when time is limited you don't want to be mucking about measuring nylon out from the spool and tying dropper knots. But cast carriers can be helpful to the everyday angler as well, especially if you have trouble tying knots or threading flies onto a leader, especially in cold weather.
Traditional knotched carriers can have the disadvantage of creating kinks in nylon, especially if a cast is stored on the carrier for any length of time, but there is a modern alternative.
Foam rubber rig winder disks
These hard foam disks were developed originally for sea angling, where they are used to store made up sea rigs, but they make excellent cast carriers for fly fishing too. You simply hook the point fly into the foam, wind on the cast, hooking in each dropper as you come to it, and secure the whole thing by putting an ordinary dress making pin through the loop at the end of your cast. The big advantage is the shape of the disk does not distort the nylon in any way. when you want to use the cast, just unfasten the loop and unroll the nylon.