Generally, starting farming artifacts is recommended when you reach above adventure rank 45 in the game. The reason is that you are guaranteed to get a 5-star artifact piece from the RNG. So, most players start saving up their fragile resin from the beginning and use it exclusively to farm artifacts when they reach the required Adventure rank. Sometimes, some artifact domains may be locked behind an adventure rank limit and not accessible in the early game. That's also a good reason to save up your resin and utilize it later in the game. Another reason players save their fragile resin is to reap better rewards at a higher world level. When you start progressing through the game and level up the overworld level, you will encounter tougher overworld mobs, and they will drop materials of higher rarity. It may happen that at higher world levels, you can get 4-star items as drop material from the bosses and standard enemies.