You can edit someone’s sperm or eggs to make changes. If you do this, then all of the descendants of that child will share those changes. If you change someone’s blood cells to be resistant to cancer (for example), then those changes aren’t passed on [somatic editing]. When you change someone’s germlime (eegs or sperm), that person not only makes decisions for themselves, but for all their potential children, forever. Then if you have multiple people who have these changes, and some of their children meet each other and have kids, you’d start to have interesting combinations of these changes interact.
It gets into pretty scary territory here, where if you’re introducing changes never seen in nature, we have no idea of how those interactions could turn out. Do you start to tell people they can’t have children together because of their parents/grandparent’s decision? Or do you try to reverse those changes in their germlines (and hopefully not introduce other changes/errors) in an attempt to fix this?
There’s an intermediate risk change one can also do, where you copy known changes that are rarely seen (but at least natural in human populations) into many people in germline editing. Some small fraction of people have a deletion of CCR5-delta32, which is thought (when you have a copy from each parent) to make it more difficult for the HIV virus to penetrate your cells. Since this mutation is in a small but appreciable fraction of people already (1-10% of caucasians), we could look at those populations of people that have the mutation and see if there are negative effects that those people share compared to control populations. If the mutation was determined to only have positive effects, then what you’re doing is in effect raising the frequency of the mutation in the population, instead of introducing something completely new.
This option is probably how germline changes will eventually happen, but most of us want to keep learning more about all of these mutations and their effects before we start changing the human species into something new.