Both jus soli (citizenship by birth) and jus sanguinis (citizenship by blood) exist more for historical reasons than because one is better than the other. Both are simply a way to try and make citizenship a more clear-cut thing, because it’s as close to being a made-up thing as you can get, especially in cases such as parents having a different nationality to the child (which is even more confusing when both parents are of different nationalities).

Jus soli is more common in the Americas due to various factors, including an incentive towards immigration from richer countries during colonial times and the various movements towards emancipation of the enslaved peoples a few centuries later, but the fact remains that neither system is any more arbitrary than the other. Jus soli is often favored because it simplifies things like immigration and asylum seeking and reduces statelessness, which is still a significant issue that affects millions of people worldwide, mostly around war-torn areas.

As mentioned in another response, enfranchisement is also a very important issue that jus soli resolves, although a significant part of it is also due to other, unrelated citizenship laws that may not necessarily conflict with jus sanguinis.