system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do storage devices have capacities that are multiples of 8?

Strictly speaking about gigabytes (8GB, 16GB, 32GB etc.)
gpt
storage is a collection of 1's and 0's (bits) and 8 bits form a byte. all the information about your files, programs and anything else is stored in this binary form. lets just call this the data. 

however just storing the data isn't enough. you have to be able to manage all this data, and how to find it all. that way when you open a file, the computer knows where to go to find all the data that comes together to form the file. this is done by giving each location on the storage device an address. and since there are SO many addresses, we actually need a way to label them all. which we do - in binary.

and because of this, a storage device has a certain number of locations we are able to reference before we run out. and without getting technical about binary (with 8 bits you can address 2^8, or 256 locations), multiples of 8 means we are using every single available location to store data. 

so if we had a 16GB USB and wanted to make it bigger, to expand it to 20GB would be silly. since we are already making more locations available to reference, we might as well use every single location and expand it to 32GB.

*thougt id add this in in case you do want to get technical:
so like i said, 8 bits mean you can address 256 locations. if we add one more bit, we get 2^9 different locations available to us, or 512. this continues... 1024, 2048, 4096..... so you can see that with the addition of each bit we get a twice as many more locations to store the data. this is why you often see 512mb, 1GB, 2Gb, 4Gb, 8Gb etc... sizes for storage!