Why does having more than one income stream make an employee, potentially better?

When an employee has just one job, all their “eggs” are in one basket – the job basket.
The employee is not the boss of the company, so are effectively handing that basket of eggs to the employer to hold. So subconsciously they know that they have to toe the line in their job and not do anything that will upset their employer unduly, because there could then be a chance that the employer figuratively "drops the basket of eggs" and gets rid of the employee.
The employer may believe that having the employee focus only on them means they get a loyal employee, but they are missing a key ingredient here. 
The employer is paying for someone to give what the job description may ask for, when in reality the employer may prefer a member of staff who can contribute "more" than the simple job description and contribute to the whole company and business moving forward.
For this to happen, the employee needs to feel very secure in their position or have multiple income streams to offset that potential insecurity.
So in a job where an employee only has one form of income, the employee may be holding something back and not be totally true to themselves, because of a fear deeply rooted in their brain. This fear is there to remind them that if they do something that displeases their employer, they will lose their job and obviously the money that comes with it.
Now if the employee has other forms of income, they tend to feel more secure and thus empowered to speak their mind and therefore “shine” when the opportunity is called for. 
This means the employer is getting a better employee that is indeed more valuable and is able to express themselves positively with fresh ideas and suggestions, rather than someone who is more cautious and doesn’t offer the same.