You should do this for apps even if you trust them.

One issue with permissions is that they apply to the entire app, including any third-party dependencies. Lots of apps use libraries given to them by advertising services -- they notoriously exploit permissions given to the app.

WhatsApp used to (still might) default to saving all photos from any chat to your phone. This led to some very surprising and unwanted photos being saved to my iPhone gallery. What a stupid idea.