Destiny 2 weapon infusion grants players the ability to raise the damage and Power Level of a weapon by infusing it with a superior one. This can benefit the player even more when it comes to weapons loaded with weapon mods, which can be slotted into weapons for stat buffs like improved reload speed or accuracy. These mods come in different rarities, just like all other gear. Legendary mods grant a bonus 5 points of power level, which may not seem like much until players begin infusing weapons.
If the player has a weapon with 250 power let's call it Weapon A but equips it with a Legendary mod, its power level will be listed as 255 (the 250 base level and then a +5 bonus) If you infuse it with a 260 weapon which we'll call Weapon B into Weapon A without the mod on Weapon A it would normally bring it up to Weapon B's original power of 260. Because the weapon you're infusing into (A) has a mod, however, the +5 bonus gets added on top of its newly infused power level, making it 265 power instead of 260.