In short, it was basically forced on them by lack of offspring.

Nerva: Succeeded by Trajan. Nerva was childless and was more or less compelled to adopt Trajan in order to secure the support of the army.

Trajan: Succeeded by Hadrian. Trajan was childless, Hadrian was his first cousin once removed. There is debate about if Trajan ever actually adopted Hadrian. After Trajan’s death, a letter, supposedly from Trajan, was produced stating that Trajan adopted Hadrian. However, the letter was signed by Trajan’s wife not Trajan himself, and it is possible that either Trajan was to weak to sign the paper on his deathbed but did know about it, or that it was arranged by his widow after his death.

Hadrian: Succeeded by Antoninus Pius. Hadrian was also childless. As a condition of his adoption, Antoninus Pius in turn adopted Marcus Aurelius and Lucius Verus, thus Hadrian actually arranged his successor’s successor similar to Augustus’ attempt to do the same when he adopted Tiberius on the condition Tiberius adopt Germanicus.

Antoninus Pius: Succeeded by coemperors Marcus Aurelius and Lucius Verus. Had issue, but only one, a daughter, survived to adulthood. This daughter then married Marcus Aurelius. Interesting, Hadrian’s arrangements were to marry Antoninus Pius’ daughter to Lucius Verus, but upon becoming emperor Pius married his daughter to Marcus instead, in effect having Marcus become the “senior” of the two coemperors.

Lucius Verus: Died before Marcus, thus leaving Marcus to arrange the succession. Had children, but all died young.

Marcus Aurelius: Succeeded by his son Commodus.

So in conclusion, as you can see, Marcus Aurelius was the first emperor of his dynasty to have a son survive long enough to succeed him. Thus, the policy of adoption was not a policy decision but forced on the Nerva-Antonine emperors by lack of surviving male offspring.