The Windows API parses input strings for file I/O. Among other things, it translates / to \ as part of converting the name to an NT-style name, or interpreting the . and .. pseudo directories. With few exceptions, the Windows API also limits path names to 260 characters.
The documented purpose of the \\?\ prefix is:
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For file I/O, the "\\?\" prefix to a path string tells the Windows APIs to disable all string parsing and to send the string that follows it straight to the file system.
This allows the use of . or .. in path names, as well as relaxing the 260 character path name limit, if the underlying file system supports long paths and file names.