Minimum of three ways 24 is used in real world

The number 24 can identify a specific object such as a train, bus or floor level. 
The number 24 can also be used to let someone know how much of something you have such as if there are 24 guests at a gathering or 24 puppies for sale.
The number 24 can also be a way to communicate.  There are 26 letters in the alphabet.  If you use numbers to spell out a word.  For example, if you want to spell out "exactly" then you would use the numbers "5,24,1,3,20,12,25" to spell it out.