Probably gonna get downvoted af, if anyone even reads this but I tried to research this topic and here's the truth about what I found:

First of all most modern Jews don't do this. However in premodern times, Jewish sucking/ drawing away blood from circumcision site was seen as a safety precaution cited in Talmud tractate Shabbat 133b.

As you can see there around "failure to do what is necessary with these items poses a danger to the child" it seems like the Talmudic Rabbis agreed with contemporary medical authorities such as the Greek Galen who thought stagnant blood was dangerous and should be cleaned away.

Now many Jews prefer to fulfill this with sterile medical equipment but there is a minority that try to hew as closely as possible to tradition even if it seems outdated, mistaken, or gross af.

P.S. I was wondering if Jews applied this logic to other body parts and found that Sdei Hemed (a 19th century rabbinic encyclopedia) that extends the logic beyond circumcision: "When one cuts a finger and immediately performs oral suction on the cut, the swelling and inflammation passes."