Q: Reuse by formatting the USB install media of OS X

select it in the Finder sidebar and press Command + I, if it is locked or set to read only you can't erase the drive. It would also have to be at least an 8GB flash drive.

What was on it?

You should always retain the original install media for a Mac. You never know when you might need it.

As it is read-only, that is all you can do with it.

Given that Staples is selling four (4) 8GB Flash drives for $12, you might consider saving the Apple drive for posterity or, as Barney-15E suggests, just in case, even if you can format it.

At the same time, if you're planning on using it with a Mac which supports USB 3.0, I suspect you'd be much more satisfied if you got a new USB 3.0 Flash drive. And Best Buy is selling a 256GB USB 3.0 Flash drive for $50.

To be sure you have adequate reserve capacity for archiving an macOS Sierra

installer with boot capabilities, you should seek and acquire a a USB flash drive

larger than the basic 8GB; that probably isn't large enough to archive older OS.

Recently, ahead of the demise of El Capitan in Mac App Store, I've download

to a suitable Mac (presently runs 10.9.5) & saved to Applications folder; since

it won't be available once Sierra occupies top-space in Mac App servers. As

my plan, nearly two years ago, was to consider partitioning internal hard drives

for trial installations of more recent OS X versions, while leaving the one this

unit shipped with, untouched; I've yet to realize that idea. But I do have 10.11.

saved; for archive USB installer, I'm looking to get larger flash drive than 8GB.

An original Apple OS X 10.7 USB key to install a system upgrade on MB/Air, is

worth a little bit more than a blank USB flash. Maybe at least $25. if you don't

have the computer and can find someone who may need it. The idea was good.

Too bad I couldn't get a USB key for these supported systems to install them

offline; then I could include one with a newer Mac when time comes to upgrade.

For special purposes where read and write of larger files to USB flash drive may

be a dedicated job, there are faster read-write speed USB flash models available.

Expect to pay a little more, or a lot more, depending on future uses or demand.

In any event...

Good luck!

And here is another reason why you should keep that original USB stick: if/when you ever sell your Mac, you are required to erase any OS or app downloaded from the app store as it is licensed only to you and is not transferable. So, you would need to reinstall the original OS in order for the recipient to be able to update it themselves.