Infusing increases the level of the piece of equipment, which continues to raise your light level. You can infuse a 290 with a 294 and boost that 290 up to a 294. But if it is a bigger gap, like 290 and 300, you might only get 296-298 out of it. I'm not sure if there is a difference between using a blue or purple of the same level. Also, I'm not sure if upgrading a blue/purple before using it for infusion fuel might also increase the amount of level increase? These are 2 things I am looking into.