A lot of projectiles in fractals can be reflected or destroyed to prevent various boss mechanics, like the expiration of Bonfires in the Snowblind Fractal or the emergence of burning fields from Solar Orbs at Arkk in the Shattered Observatory Fractal.
Below is a list of currently used projectile reflect or absorb skills and traits:
Certain mechanics can be bypassed by using block, Invulnerability or Aegis skills. Those ward against any attack (including projectiles) and usually prevent the application of conditions and control effects. Note that some abilities are unblockable!
Below is a list of commonly used defensive skills: