How do tampons work and are they a necessity to carry them if they can just put one on in the morning?

I have been learning about girls since I got a girlfriend.

https://www.reddit.com/r/NoStupidQuestions/comments/5m8f1f/why_do_girls_carry_purses/

https://www.reddit.com/r/NoStupidQuestions/comments/5kmimf/what_exactly_are_periods_why_does_my_14_year_old/

How do tampons work exactly, do they just collect the blood, wait for it to harden, then stick to the tampon, then bin it? I have been told tampons are a necessity, but why do girls carry them? Can't they just predict the day they need it, and put one on in the morning and take it off at night ?

Thanks for any information.