Many trainers and coaches preach the importance of the lowering portion during lifting as they believe it to be more important for growth. The reality is that many of the studies that show this to be the case used greater loads during eccentric-only training compared to concentric-only training. Since normal training involves the same weight during both the eccentric and concentric portion, the eccentric growth benefit doesn’t occur! If you want to train true eccentrics to maximize growth, you need to train with very heavy weights and use the lowering portion only.
Source: Schoenfeld, B. J., Ogborn, D. I., Vigotsky, A. D., Franchi, M. V., & Krieger, J. W. (2017). Hypertrophic effects of concentric vs. eccentric muscle actions: a systematic review and meta-analysis. The Journal of Strength & Conditioning Research, 31(9), 2599-2608.