One example: I use RStudio every day for work, which is fully open source and developed by a company seeking to make a profit.

Recently I got around to making web applications using R, made easy by the people at RStudio. Again, everything is open source.

However, running a server is a lot of work, and something I couldn't really do in the context of my job even if I learned how to. So we instead pay RStudio an annual fee to host the application I built, and we are of course happy to pay. :)