why do clarinetists have like a case where they have multiple open reeds?

i’ve had a clarinet for 6 years (notice how i’m not saying i’ve been playing for 6 years or have 6 years of experience) and i’ve realized that there’s clarinetists that have multiple open reeds, usually in this case where they just like… choose one for the day?? i don’t fully understand why, is it because of mold? i’ve kinda just been keeping the reed in the little case it came in and only having one open. i’ve looked for answers online and i’ve gotten no answers, and no i’m not asking my classmates that do this im a scaredy-cat. please explain this phenomenon!!

• love, scared clarinetist