questions for hellene

Hellene
whenever i have time i like to check through the forum and read some other people progress diaries and look forward to your recommendations . I am finding i learn stuff i didnt know about that i can apply in our piano learning .
For example : 1. Don't master piano pieces for sight-reading. It doesn't help you to develop eye-sight for lines and spaces. ??
can you explain more about this please .
thanks
viv