If you are looking for a job to work part time with your current full time job I would recommend having a brief meeting first with you're boss. Even if you have made them aware of working it may still be seen in a bad light. There are some important factors to remember when approaching this situation as well as understanding why they need to know.
Know Why Your Company is Worried
Employers typically don't see any advantages to their employees working side jobs, but it can be beneficial for both employer and employee, stimulating creativity, motivation, and new ideas. The key is to keep the focus on your current position:
Put the company first.
Most employers want to hear that you'll continue to put your job at their company first. Assuage your manager's concerns by letting her know you won't work at your second job during office hours and that you'll still be able to work overtime during periods of heavy work. Show your commitment to your current position as a priority.
Lay out how you'll remain effective.
Your boss doesn't want you to be overwhelmed and fatigued just because you're working multiple jobs, so lay out the strategies that will allow you to remain just as effective as you are now. Don't talk about the new job; discuss how you'll continue to rock your current one.
Keep quiet about confidential information. A small number of companies will be concerned that you'll leak confidential, in-house information, particularly if your side job is utilizing the same skills that your existing job does. If you can't get permission to work in the same vertical, try a side job in a different arena all together.
Act as a good representative.
What you do off-hours can seem like none of your employer's business, but it is, especially if what you're doing could be deemed offensive to your employer's customers. Make sure that whatever side job you choose won't put you in an awkward position with any of your company's clients, partners, or customers.
These tips should get quick approval to take on the side job. It may also give the person in question an insight into why the company may be interested in this information.