There are many reasons why getting a 2nd income stream might be a good idea. If you have a family, have just gotten your degree, been laid off, been off work due to an injury or accident, you may have accumulated a large debt and the full-time job you have just does not begin to allow you to start paying on that debt and have money to live on. You may also just have a part-time job. Even if your spouse goes to work there still may not be any left over money to put away for retirement, emergencies, vacations, etc. These are legitimate reasons when a 2nd income stream might be better.
Starting a 2nd income stream does not mean that you have to go out and get another full-time job or even a job out of the house. There are many different online jobs or jobs that you can do at home to start that 2nd income stream. Two of the many different 2nd income stream jobs you can start include starting a computer consulting business which means helping people find computer systems match what they need a computer for and minor repair jobs, or start a blog and earn income from the advertisements on your blog.
Another reason it might be feasible getting a 2nd income stream is that you are self employed and have no retirement benefits that you would have if you were not self employed or you work for a company that does not offer retirement benefits. Having a 2nd income would help you build a nest egg for retirement. In addition, if you are retired this extra income would help to supplement your retirement and help you not have to live from one month to the next with no extra spending money.
If you decide that you want to take an expensive or extensive family having a 2nd income stream would help you reach that goal much faster. You could also use this extra 2nd stream income to build up an emergency fund. If you are starting a family and your wife wants to take time off to be a full-time mother you may need to have a 2nd stream income to supplement the income she was bringing in before she took a leave of absence from her full-time job.
As you can see there are many different reasons why getting a 2nd stream income would be a good choice. Just make sure whichever 2nd income stream you choose is profitable.