Many companies were not able to develop software and asked us to outsource their software. Sometimes the reason is that the company (often it's a startup) can't or doesn't want to hire in-house developers who work on a long-term basis. They need to work for a limited time and after that, just use the developed app or the system. Sometimes it's another agency or a middle-man who can't perform such qualified work by himself and looks for specialists who can do it. Sometimes the company just doesn't have expertise in a critical part of the software, but they can do all the remaining parts. For example, we have a video processing project where the crucial part is low-level video operations, and it's done in C++. But in-house customer developers are node.js developers who are implementing a web service. They do the critical part, developed by us, and use its functionality from their node.js code.
For this reason, our customers require to combine several technologies or programming languages together. They reduce the cost of development because they need to do the critical part with our help.