I would argue that the product you see on the outside (Stripe) is a reflection of the work that gets put into internal only projects. In many ways, this sort of "build the things that people want to use" culture causes people to experiment, to hone their skills, and to develop.
Stripe landing pages almost always get tons of support on Hacker News. This isn't because they're just a good payment processor -- it's because their landing pages are unlike anything else in the industry. They could have slapped together a static site in Jekyll with custom styling and called it a day. Instead, they labored over a landing page that feels well designed with immaculate attention to detail.
Did they have to do any of this? Not really. But it shows on the outside -- and it works. Given that the world is filled with design that just copies Apple and Google, supporting creative R&D is a huge plus for them.
Internal tools are the best way to do that -- they let people experiment and learn without fear of consequences on the outside.