To put it simply: developers write code in C++, but they don't want to reinvent the wheel every time they need their software to perform a standard task (like opening a file or drawing a window). They use standard libraries to do this.
When you download a program, it expects your computer to already have these standard libraries installed. The Microsoft Visual C++ Redistributable is the package that installs these libraries. Without it, the software cannot run.
Even with the offline installer, you might face issues. Here is how to solve them:
Because multiple Visual C++ versions (2005, 2008, 2010, 2012, 2013, 2015-2022) are often required, many advanced users download an unofficial community pack. However, for security and stability, Microsoft recommends installing only the official 2021 (14.x) offline installer plus older versions only if your legacy software demands them.