The Question as written, is a little bit leading. Constantine the Great converted to christianity on his deathbed and thus his motivation was not earthly and personal.
He was baptized, and received his first Communion on his death-bed, when sixty-four years old, and is remembered as the first believing monarch.
Constantine likewise as I recall converted to several religions at the same time on his deathbed although I can't source that, so his motivation likely wasn't exactly in line with Christian values either. It was more covering his bases in case their was a Christian afterlife, a jewish afterlife, etc..
If the question is why did Constantine in 313 AD, issue the Edict of Milan decriminalizing Christian worship.. It was certainly not to loot the christian temples, which is absurd since christianity as an outlawed underground movement had no such public temples or riches. As an outlawed persecuted religion such posessions would have long ago been confiscated if they had existed. Only, those possessions had not yet occurred, they would come as a result of being legitimized over centuries.
Constantine's decriminalization was more about forming an alliance.
Although the Edict of Milan is commonly presented as Constantine’s first great act as a Christian emperor, it is unlikely that the Edict of Milan was an act of genuine faith. The document should instead be seen as Constantine's first step in creating an alliance with the Christian God, who he considered the strongest deity. At that time, he was more concerned about social stability and the protection of the empire from the wrath of the Christian God than he was for justice or care for the Christians. The edict is more indicative of Roman culture’s obsession with seeking the gods' intervention than of Constantine's or Licinius's religious beliefs.
313 AD was a data point on a long line of fascination with Christianity for Constantine. Constantine's interest in Christianity wasn't wealth, it was marshal. Constantine came to power by defeating rival roman emperors by rallying Christians to his cause. He as many Romans had witnessed Christians going to their deaths across the empire singing and rejoicing; and Constantine had thought to himself.. If I had a few legions of those guys I could deal with the fractured empires troubles. So Constantine began to recruit Christians for his army. The battle of Milvian Bridge also 312 AD, is an example of Constantine use of Christians as his warriors. Fighting under a Christian cross, and giving all credit for the victory to the Christian God, and representing himself as the chosen one of God to lead the empire. Only Constantine was still a Pagan when he did all these things.
The flaw in Constantine's plan was that before he could use christianity to unity his empire; first Constantine had to unify the Christian churches. He had to create a unified Church a catholic Church. The problem was in 325 AD christianity was a series of loosely affiliated underground religions which had survived for centuries of Roman persecution. Christians were put to death across the empire as they were discovered, so christians had become skilled at hiding. This hiding didn't allow the free flow of ideas. Christianity was thus a fractured religion made up of many different beliefs, each city lead by a bishop with his own doctrine which may or may not agree with the bishop who had set up shop just over the horizon in the next city. These bishops traditions and doctrines had evolved independently as their was no authority and limited contact with other branches of christianity. So Constantine couldn't use christianity to unit the empire until he united christianity. On this note, the pagan Constantine the Great called the first christian church council, the council of Nicea. Nicea being a suburb of Constantinople, Constantine's capital. He called the bishops of christianity to him. He personally presided over this council, and personally set the agenda... What he wanted and what he got was a unifying statement of belief that would unite all the different churches into one united catholic Church.. catholic meaning unified and being written into the creed. That statement generated at Nicea is today the Nicea creed which remains the most widely used christian profession of beliefs across Catholicism and much of the Protestant churches. I've even heard the Mormons use it.
Among Constantine's other innovations was the use of Christian relics. Body parts of martyrs or saints or even items which had come in contact with Christ himself. Constantine collected these totems and used them to court favor from christian leaders, or demonstrate his great knowledge / wisdom about christianity. This was the start of centuries of fascination with relics by the christian church which would only be curbed by Martin Luther's reforms.