The source code details the way a program works (it is effectively what programmers write). Without it no one can understand how the software is built.
It does not matter if you can't read code: whether or not it is available directly affects any user.
No one can look inside
Windows comes without its source code. More than that: all users must abide to the license term that says:You may not reverse engineer, decompile, or disassemble the software.
As such, it is illegal to work out how Windows or Microsoft Word are designed. It is even illegal to try. You are also forbidden to modify the program for any purpose.
This restriction in the license makes sure that Microsoft remain the sole organisation that understand how their products work. Windows is very much like a car that only the original manufacturer is allowed to service.
Insecurity means business
Whenever a flaw in Windows is discovered, it is exploited, which results in trojans, viruses, spyware and the likes. Such nuisances delight the Windows security industry, including Microsoft, which develops various anti-virus protections with subscriptions.
It is a lucrative process to release flawed software that no one is allowed to change, and then selling protection services over it (anti-virus software doesn't correct flaws: it merely prevents viruses from exploiting them, if it is active and updated).
Microsoft will today happily sell you their own protection over their own insecurities – it sells the poisoned apple and its antidote, separately.
You can trust free software
It comes as no surprise that proprietary software is severely lacking in terms of security, compared to software whose source code is freely available (including GNU/Linux).
Free software means programmers can change the code to repair flaws. It means you can hire someone to really check how secure your software is. It means you can benefit from the contributions of a world-wide community to improve safety and reliability. Fifteen million users run GNU/Linux without an anti-virus, in complete safety. And the servers behind search engines and banks run on it too.
Source code is the recipe for software. How could one improve on a meal with too much salt, if forbidden to look at the recipe used to cook it?
Using Windows and Office requires us to not ask nor search for the source of the software. Only Microsoft developers can modify your program.
Which would you rather trust: the package you are forbidden to study, or the package with the recipe?