1. Standalone, tiny: one app, 32kb or 44kb, depending on if it's a 32 or 16 bit system.
2. Portable: independent, so you can bring it on a flash drive, or even e-mail it.
3. No config needed: you just run the program, and it does what it's meant to do. You don't need to tell it what files to join, it just joins whatever parts (ie. x.001, x.002, etc) are in the folder that it is in. Also, if there are two different types of split files, like "monkey.zip.001" and "freeware.zip.001", it will automatically combine the correct files, so it's not like you have to be worried about it joining things it shouldn't.
You might be wondering, "What is the practical application of this? In the olden days, they had to split files to fit on floppy disks, but now I have all the space I need." One word, my friend: e-mail.
Every e-mail service has an attachment limit. If you have a file that is over that attachment limit, there are two different options you can do: upload it to some site that makes the other person wait for 2 minutes before they can download, or split it. The latter is much easier.
So after you split your file using a file-splitting software (such as HJ-Join's cousin, HJ-Split), send the pieces, one by one through e-mail. Then, send HJ-Join, and just tell the person to run the program, and they'll have their file. You don't even have to be computer literate, all you have to do is put the pieces and the program in the same folder!
So that is the main use for file splitting and joining. The thing I like about HJ-Join is that it's small, and easy to use. And it works with any split files. It's one of those things where you are probably going to need it one day, and it's only 44kb, so why not have it?