Italo Vignoli makes a great point on his blog about the use of fonts. He explains that proprietary software like MS Office uses proprietary fonts by default.
Because of the way they are licensed, they can’t be bundled by other software. That means substitute fonts with different characteristics have to be used. As a consequence, other programs trying to open documents they create — no matter how otherwise interoperable the file format handling becomes — cannot reproduce the same visual appearance or layout since they don’t have the fonts.
The solution to this is open source fonts. They can be freely bundled with software like LibreOffice and thus the documents using them are much more likely to render correctly on other systems.