If you are including multiple libraries, some may have the same namespace. ‘Using’ a namespace could cause an error, the program to crash, or to not work as intended if the libraries have namespaces that are the same but do different things. Like namespace “X” in one library does one thing, but there is a namespace “X” in the other library that does something entirely different. “use X” isn’t specific enough then for the computer to understand what you’re wanting to do.
The easiest comparison I think would be like having two people with the same name in your Contacts list. They both are different, but because they’re both listed as “Joe” you’re not sure which to call.