Despite the recent progress discussed in Sec. 6, we still do not clearly understand the mechanisms of collective vibrational strong coupling and their modification of reactivities, or the available mechanisms that can take advantage of the collective coupling of forming polariton that changes photochemistry reactivities (except for charge transfer). In Sec. 6.1, we review works that elucidate how collective coupling can modify photophysical properties, such as energy/carrier transport, population dynamics, and linear and non-linear spectroscopy. On the other hand, in Secs. 6.2-6.3, we show how photochemical reactivity may be modified by collective effects. Finally, we discuss the mysteries of modifying chemical reactivity in the vibrational strong coupling in Sec. 6.4 and illustrate a few theoretical works that have attempted to address this issue. This cutting-edge research has many opportunities for both theorists and experimentalists to contribute and discover new physics. In this manner, much work is needed from both sides to demystify these collective effects and unlock their potential applications.