When we can attach the file through mail -a or mailx -a then what is the special advantage uuencode offers.
Maybe you have a mailx with a -a option, but you're writing a script that should be portable to an older unix without it.
Maybe you just have an old script you're maintaining.
Maybe you're sneaking something past a mail filter that blocks MIME attachments.
Maybe you're actually making productive use of the permissions in the uuencode begin line.
Maybe you're sending to a recipient that finds uudecode easier than MIME parsing (such a recipient would probably be a unix-based bot of some sort).
It's maybe an easy to understand educational example for people learning about pipes.
In general, if you have no technical or personal connection to the pre-MIME mail system of the early 90's, nobody expects you to use uuencode, at least for mail purposes. You might still use it if you have to cram a file transfer through some other text interface, like a serial console.