Because people can justify paying for a service if it helps them conduct their business and make money.
If you help people make money you can also charge a higher cost for your web app. Its not reasonable to spend $49 a month sharing your videos, you wont find many people willing to shell out that kind of money on a recreational activity. But it is reasonable to spend $49 a month running your business. If you build a web app that makes people money they will pay more, a lot more, to use it.
Also if you’re helping people make money, there’s another side benefit. When you’re in charge of running a business you usually want the best equipment. For example, if you can afford it you would upgrade your typical office chair to an Aeron. Its somewhat similar when it comes to web apps. Entrepreneurs won’t settle for a standard plan when they can upgrade to a premium plan. Choosing a premium plan over lesser plans intrinsically shows you and your employees that you’re serious about business.
Its like choosing Windows professional edition over Windows Home edition. A lot of business’s could get away with using Home edition but they’ll purchase professional edition anyway. Business owners don’t want their employees seeing HOME edition when they boot up, they want their employees to see their running professional. There’s also an element of human nature, rather than doing more work its easier to show your dedication by purchasing better equipment.
Expect the same with your paid plans. If your application helps people conduct business your users will be more receptive to upgrade their plans to a higher cost premium plan. If you have an app that people are using recreationally it will be much harder for you to convert them to higher paying plans.
So if you want to build a web application that makes you money, then build a web application to help make others money.