Existing users can keep working
An API can add an optional request field while still accepting older requests. A new app version can read data saved by the previous version. Both changes extend the system without forcing existing users to adapt.
Forward compatibility means older software can handle newer data or behaviour, often by ignoring fields it does not recognise.
Check what users rely on
A change can compile successfully yet still break clients that depend on the order, timing, or meaning of a response. Testing with existing clients helps catch these problems. If the old behaviour must end, a deprecation period gives affected teams time to adapt.