It didn't really. The practise of adopting an heir was the standard because the reigning emperors simply didn't have (male) heirs of their own. Vespasian, who came out victorious after the turbulent "year of the four emperors" (68-69 AD) actually had two sons. Titus and Domitian both succeeded their father, with Titus even co-reigning with him for a few years. However, due to the nature of Domitians... psyche? - he was assassinated in 96 AD and became one of the few emperors who was issued the damnatio memoriae.

Because of that, the senate opted for the previous practise of choosing a successor to him, the choice which fell on the aging Nerva, a respected senator. The following four emperors: Trajan, Hadrian, Antoninus Pius and Marcus Aurelius (plus Nerva), are called "the good emperors" because they were all chosen for their qualities rather than their blood. Even if the Julio-Claudian emperors were all adopted, they were still part of the Julio-Claudian family. That changed after 96 AD.

Only one of these five had a legitimate heir to succeed him - Marcus Aurelius. Commodus is, thanks the movie Gladiator, fairly well-known in the general public today and there is little doubt that he was about as bad an emperor as the movie portrays. He was of course also assassinated, and was succeeded by Pertinax - who tried to reform the praetorian guard and was killed by them for trying to do so. He was then succeded by Didius Julianus, who was to set a new practise for becoming emperor. Didius Julianus bribed the praetorian guard to put him on the throne after the assassination of Commodus (which differs from when the guard put Claudius on the throne on their own volition after killing Caligula).

Didius Julianus was highly unpopular, not in the least because of the way in which he became emperor. However, everyone did not accept him as the new emperor and instead supported another man, a highly competent general called Septimius Severus. Severus could have been a new Vespasian (who was very popular) and did in fact become emperor himself after sentencing Didius Julianus to death. Severus was himself a very competent emperor and ruled for almost 20 years. He had two sons, Caracalla and Geta, and he decided that their were both to succeed him and rule the empire together. However, these were not two loving brothers, they hated each other, and it was only a question of who would kill whom. Turns out Caracalla was the one who first came to action, and he ordered his brother killed - as well as ordering the damnatio memoriae to be executed on him.

To be fair though, Caracalla wasn't actually a bad emperor, some things he did were in fact quiet good; but what he's remembered for is the killing of his brother and the persecutions of different peoples all through his reign. Caracalla is actually very similar to Caligula when you look at their stories, even though almost 200 years separate them.

Either way, Septimius Severus and Didius Julianus started a new practise that would allow generals with great power to ascend the imperial throne, sometimes through bribing the praetorian guard. This started a period we today call "the soldier emperors".

Apologies for the long reply to a seemingly straight-forward question, but as you can see, the answer isn't as simple as one might think. On three occassions during a period of ~140 years, a practise was about to be set with succeeding biological children. All three of them failed because the heirs were nowhere near as competent as their fathers.