Because of the way DLL files work, many of them are included by default in any windows installation. So, when you install a program, it assumes that the information it needs to run is already present on your computer. If a particular DLL needed for operation is missing or corrupted that program will no longer work. It will give a DLL error that you see on your screen.
In most cases, DLL errors appear when programs or files are corrupted by viruses or other malware, or even removed by your anti-virus program after such an infection. Other situations may appear when a file is removed during uninstallation of a program or when removed manually by accident. But it could even be as simple as you trying to run an older program on a new computer, where the old DLL files that the program requires are not included by default in your new Windows installation. The opposite, trying a new program on an older computer is also a possibility for the same reason.