Unnecessary information can take away from the sharpness or clarity of the sentence you are writing. The incidental or secondary details may be important information that needs to be stated somewhere, but placement is key. You want to clear up space for the main idea.
Amateurish writers have a tendency to pack sentences with as much information as they can, and don’t really think about the extent that this may obscure their main point. They add things like days of the week, or percentages.
For clarities sake, leave less important, but vital information out of the point you’re trying to make. Stick with the main idea. You can always provide additional information later. Remember sentences lose power when they are too wordy or difficult to understand.
Just something to think about 🙂