How should I answer her?
All that costs money. It might be more than you have in your checking account. That's where investors come in handy.
So, you want to strike when the iron's hot, and take advantage of that brand. You want to get Flappy Bird sequels coming out, you want Flappy Bird stuffed animals for children, Flappy Bird backpacks, etc. You want these things happening when the game is in its peak, and if you try to do everything on your own, you won't make that deadline, and Flappy Bird will be quickly forgotten. So, you get investors, bring in millions, hire staff, and start pumping out Flappy Bird everything to keep it in the spotlight and to grow the business. Or you start doing spinoffs, Wiggle Frog, etc. Either way, you need money to get things moving... fast.
Secondly, you need to fight off competition. This isn't a huge concern with Flappy Bird, but let's say you make a simple app that let's you post a few photos of yourself, and others vote on your best picture. You launch, it's a huge success, and you have practically zero expenses. Now, you're going to have a dozen competitors fire up overnight, and some of them will have more talented developers, some of them will have more free time, and some of them will have huge budgets. They want to take your crown, so you need to stay one step ahead of them. This is where investors come in. The money goes into polishing the app further, turning out new features, or related apps.
If you want to market your app to people, you'll need to pay for ads.
However, most investors aren't really going to be interested in funding something unless they're convinced that there's a good chance it can make a lot of money. If all you want to do is create a small-scale project that doesn't need to support a lot of users, you could fund it out of your own savings.
Also you need to think about the time spent and income being made. Getting investments mean you can treat it as your full time job and not worry about dipping into your own savings after your 3rd month of advertising without getting much return.
It really depends on the app and if there's market demand ($) for it beyond just users.
If it's the first app, learning how to make money is more valuable on a smaller project than building something big and having less of an idea.
