You can always normal summon "Level Warrior" as a level 3 monster. You may only normal summon "Level Warrior" as a level 2 monster (using "Level Warrior's" effect) when there are no monsters on the field. You can only Special Summon "Level Warrior" using its effect when your opponent has a monster(s) on the field when you don't. A very flexible card when Synchro Summoning.
Its effect that Special Summon itself as a Level 4 Monster is perfect to conduct a Rank 4 Xyz Summon provided that you Normal or Special Summon a Level 4 monster after this card is Special Summoned with its effect.