Altering time lines is one of the most powerful tools in this Galaxy (and likely always has been)

How is it primarily used?

The most common type of time alteration is to prevent the birth or births of people who cause planets to blow up or become uninhabitable before they are born. In this way by eliminating one person through time alteration you can prevent whole wars or nukes or other things that destroy civilizations.

You might ask why people like Stalin, or Mao or Pol Pot or Hitler or others who killed each one millions of people in their times were not eliminated before their births?

However, that isn't the way things work. The ones that are taken out of time usually have caused the most damage of anyone (in other words billions or more people have died before they eliminate someone important through time alteration. To eliminate someone from the time line civilization itself usually has been destroyed in one way or another.

Why is it done this way?

Altering time lines is sort of like pruning trees. In some ways it is dangerous to do this on some planets because maybe the time trees are not that healthy or resilient on  Those planets or nebula or stars or wherever these things might occur.

So, altering time is usually done sparingly if possible and some places depending upon the cultures are left alone entirely simply because it isn't practical for many reasons to alter time there.

However, there is a wildness to the time tree that serves earth and so we are fortunate to have a very strong and resilient time tree here. However, disappearing people out of time is still done sparingly and usually only after they seriously harm civilization retroactively. Even then often an Android is put in place instead of them to live their lives so they are not missed by the time line. But, the android doesn't end civilization when it is put in place instead of the human being it replaces.