Much like barcodes are a bit of a hobby, magnetic cards have been for a long time. I made my first mag card reader using a Sony walkman cassette head mounted on a block of wood, over 30 years ago...
Back then magnetic stripes were the main way bank cards worked, even for cash machines, long before we had chips in cards. Oddly, they are still quite common in the US but I understand chip and PIN is catching on. They are also used for some door entry systems.
Well, on a whim, I got one of these :-
Of course, really, most people have no need for a card reader, and even less use for a card writer.
Now, in my case, we sort of do. We have a nice card printing machine, which will also encode mag stripes on cards. The driver code is written by me (as had no linux drivers). We sell printed cards, including encoding mag stripes on cards for customers. We used to have a card reader and it seems to be missing, hence my buying a new one. It is useful for checking things if needed. But I thought I'd order one that writes, why not? I got an MSR Pro USB reader/writer. Seems easy to use, and very flexible.
To be honest, unless you have a mag card based control system of some sort, a door entry system, or maybe handling mag cards is part of the business (like us), you probably have no "legitimate" use for a reader or writer.
Of course there are probably fun uses for this, and also not so legal uses, especially if there are places that only use the mag stripe in some way for bank cards. These are few and far between, but I noted when in the US they not only used the mag stripe but also print the "name" from it on the receipt.
I am not sure I have the nerve to do it, but I could, for example, recode a card so that instead of track 1 containing ^KENNARD/ADRIAN^ it could contain ^SERVICE/INCLUDED^ so that in the US it prints that on the receipt just to confuse them. Would that be "legal"? I have no idea (and even less so for US law). The card remains the issuers property but this is not "damaging" or even defacing their property, and it can be undone by re-writing the original coding. It is not done in order to defraud anyone (even in US, tips are supposedly optional). So might even be legal. Of course there may be specific laws covering this (there is a law on changing ESN in mobile phones, would you believe!). That said, I am not sure I'd want to get caught doing it...
The more dodgy thing to do, is to recode the other details, copy someone's card mag stripe to another card. Now, these days, with chips being used rather than mag stripes, it is not going to work. If done to defraud someone it would be very illegal.
So whilst this is a fun toy, it is really only any use for things like checking we have correctly coded cards, and debugging the code that drives the card printing machine. So I would not suggest you rush out and buy one...
P.S. First thing was packet dumps from my machine whilst running the card reader s/w to double check it was not sending every card I scanned to China.