Imports are unused error.

[nkhar]

I cloned the project and after Gradle finished syncing, some parts of the code were red. These were from classes that were from unused import statements. I want to know what may cause such an error?

https://stackoverflow.com/questions/21100688/android-studio-suddenly-cannot-resolve-symbols
File menu, then Invalidate Caches/Restart... then Invalidate and Restart solved the problem.
However I am still curious why imported statements gave error which said that they were unused.