Why is variation beneficial to the species but not necessarily for the individual?
Variations are beneficial to the species because they provide a chance for survival when environmental conditions change. Niches can change due to reasons beyond the control of organisms, such as temperature changes or water level variations. If a population is suited to a particular niche and it is drastically altered, the population could be wiped out. However, if some variations are present in a few individuals, there would be some chance for them to survive.
For example, if bacteria living in temperate waters experienced increased temperature due to global warming, most would die, but the few variants resistant to heat would survive and grow further.
However, variations are not necessarily beneficial for the individual. Every individual organism cannot be protected by variations. Some variations might be so drastic that the new DNA copy cannot work with the cellular apparatus, and such a newborn cell would simply die.
Explanation
The textbook explains that niches can change due to factors beyond the control of organisms (temperature changes, water levels, meteorite hits). If variations are present in a few individuals, they have a chance to survive when conditions change. The example of heat-resistant bacteria surviving global warming illustrates this. However, the text also clarifies that 'Every individual organism cannot be protected by variations' and some variations can be so drastic that they cause death of the newborn cell. Thus, variations ensure species survival but may not benefit the individual.