Because of the problems those non-standard characters can cause!
Examples: Some people like to use file names such as "#3Problem.htm"
THIS IS A TERRIBLE IDEA!
Web browsers interpret the "#" character as a bookmark and cannot properly determine what the hyperlink means.
How to tell if you have a misformed hyperlink.
If you put a space in a file name, when you hover over that hyperlink the space will show up as '%20' in the path name.
Other non-standard characters show up as other '%...' values.
The '%' sign indicates that the following numeric values are Hexadecimal (base16)
This is a good rule to live by not only for file and folder names but also for:
1. variable names in programming,
because some programming languages do not allow it.
2. .exe file names in programming, .xls or .xlsx files...
because browsers will not download and run these program files!
3. field names in databases
because some databases do and some do not allow it, and if you start using spaces you get into problems when you move data and tables between them
Access does allow spaces, SQL server does not
4. VirtualPC will not recognize vfd files that contain periods in the file name
5. Adobe Connect will substitute a '+' for a space in a file name. The file cannot be downloaded, so file sharing becomes impossible.
6. Excel will not accept spaces in Named Ranges.
7. Microsoft OneDrive does not like non-standard characters and will issue error messages.
Beware that Microsoft programs such as Visual Studio can generate file and folder names that use non-standard characters which trigger OneDrive errors.
Microsoft was contacted about this and wanted to charge us to fix a problem that they created!
If you run into any more let me know so I can add them to the list.