For survival during unfavourable conditions / Fusion of gametes helps to pool their resources for survival (hunger theory of sex) / Zygote develops a thick wall that is resistant to dessication and damage, undergoes a period of rest before germination.
Detailed Answer :
Organisms such as fungi and algae switch to sexual mode of reproduction during adverse conditions because sexual reproduction brings variation into the individuals, some of which might help the individuals to adapt to the changed conditions and survive. This ensures the continuity of species.
