CRISPR has incredible potential for eliminating genetic diseases, such as sickle cell anemia, muscular dystrophy, AIDS, and cystic fibrosis. There are more than 75,000 mutations to the human genome that can cause genetic diseases, and CRISPR could potentially be able to fix all of these.
CRISPR has not been shown to have significant negative side effects, so it seems that using CRISPR to cure genetic diseases would massively reduce human suffering. For this reason, many bioethicists hold that it would be ethical to use CRISPR to eliminate genetic diseases. 
Allowing CRISPR to be used to treat diseases has been relatively uncontroversial. A joint committee formed of members of the National Academy of Sciences and the National Academy of Medicine decided to endorse research towards genetically modifying embryos to fix genetic mutations that can cause serious diseases. However, on the subject of modifying embryos for “enhancement,” the committee decided that doctors should not proceed with any treatments that do not focus specifically on curing genetic diseases.
Many debates on CRISPR draw a firm distinction between using CRISPR therapeutically and using CRISPR for "enhancement." "Enhancement" refers to how CRISPR could be used to select for certain physical characteristics that have nothing to do with the individual's health. For instance, with further development, CRISPR could potentially be used on an embryo to ensure that the person has a certain eye color, height, or other traits.
Many have supported the claim that modifying embryos for "enhancement" is unethical based on its close connection to eugenics. Eugenics is the practice of arranging human reproduction to increase the occurrence of certain traits.
If we allowed CRISPR to be used to select certain traits that have no bearing on health, we could become a more discriminatory and unequal society, which many ethicists believe to be an unethical outcome.
If a culture decided that certain features were desirable over others, and it became easy for the wealthy to get these features through the use of CRISPR, this would inevitably contribute to greater oppression against those in lower socioeconomic standing. Some ethicists have also pointed out that if we allow CRISPR use for “enhancement,” it’s possible that enhancements might become necessary. For instance, if CRISPR could be used to make kids smarter, then people might start to think that only kids who are genetically enhanced ought to be born, since that will ensure a more competent new generation. This would obviously be a huge ethical issue, since it would restrict people from their human right to reproduce, particularly targeting those in lower socioeconomic standing.