What is source code ?
Source code is the blue-print of software.
Source code is considered intellectual property. Computer programs or software cannot be created without source code.
Source code is something that is generated by the programmer from his own skills and knowledge. Just as a cook is unwilling to share the recipe of his signature, a programmer will not share his source code.
What are the threats ?
Cybercriminals of hackers are interested in stealing source code.
Their objective is to sell it to competitor companies, often after making some minor changes to the source code, to make it appear original and different. Criminals may also hold the owner company ransom for monetary grain.
Some people just do it for fun.
What are the risks ?
In all the mentioned threat probabilities, the end result is huge financial loss adding to loss of time and efforts invested in coding and designing the fully functional software.