People may use different versions
An app update may fix problems or add features. Older versions can remain on people’s devices, so several versions may use the same online service.
A version number such as 4.2.0 identifies a release. A separate build number helps the app store tell uploads apart. The minimum supported version is the oldest version the online service still accepts. If it rises to 4.0, people using 3.x may need to update before they can continue. Some older devices may not be able to install that update.
Required updates can disrupt people
Teams may stop an old version from working because of a security problem or a major change to the online service. A required update can interrupt someone mid-task or prevent them from using an older device. Teams can keep older versions working where practical (backward compatibility), warn people in advance, and make updating straightforward. If people cannot update, they need a clear explanation and a way to get help.