If your connection gets a bit weak, and there's a stronger signal available, it switches. Switching between IP addresses (which will happen if you switch between mobile data and wifi - in either direction) will break the connection, so if you're downloading a file when it happens, you'll have to start over. (It's fine for web browsing, because browsers operate in disconnected mode. They get the page, then disconnect. So if the next request is for a link to another page on that site, and it comes from another address, the site doesn't care.)
It doesn't care if the signal you're using gets weak for a fraction of a second (which internet apps can handle), it switches.
The phone normally works better with it disabled.