SMB is a file sharing protocol and, as such, it is sometime left open to the internet for, well, sharing files.
However, this is a very bad idea. Compared to simpler protocol as FTP or WebDAV, which basically have very small GET/PUT interfaces and are entirely implemented in isolated userspace processes, SMB is a much more complex protocol, deeply integrated into core Windows services.
The more complex nature of SMB (and it’s very low security/integrity until at least version 2) means that many critical flaws were exploited, and its tight integration with Windows means these exploit were very dangerous.
So, no, do not open SMB to the internet