In the early Roman Empire the Emperor was usually not a huge participant in Military command. Augustus had a man named Marcus Agrippa (https://en.wikipedia.org/wiki/Marcus_Vipsanius_Agrippa) that took command of his armies including during his civil war against Marc Antony. Most of the Julio-Claudian dynasty were more administrators than Generals. Caligula and Claudius did command troops in the field but never fought any major Battles.

During the year of the four emperors all four had military experience and with the exception of Otho they all gained the throne by commanding armies against the others. During the [Flavian Dynasty] (https://en.wikipedia.org/wiki/Flavian_dynasty) founded by Vespasian the emperors did lead troops but it wasnt until Trajan or Marcus Aurelius that the Emperors started commanding troops in major wars.

After the death of Alexander Severus in 235 BC the emperors became first and foremost Generals because of the frequency of Civil war. The emperors needed to be at the head of the biggest and best armies in the Empire to protect their throne.