Especially on the long down hill sections, you will protect your joints and also save your muscles from unwanted fatigue if you use them!
The trail to Everest Base Camp is not a straight forward trail.  There are rocks, boulders, hills and rivers.  Having trekking poles, and being comfortable using them, will help protect your joints and muscles each day.  Especially on the long down hill sections, you will protect your joints and also save your muscles from unwanted fatigue if you use them!