* Developer's Role: Developer (hydrogen peroxide) *opens* the hair cuticle, allowing the lightening agent to penetrate. It's essentially the catalyst.
* No Lightening Agent: Developer alone doesn't contain ingredients that lift color from the hair. You need a lightening agent like hair bleach (usually containing persulfates) or a high-lift hair color to actually lighten the pigment.
* Damage Without Benefit: Applying developer alone will likely cause some damage to your hair (due to the cuticle opening) without providing the desired lightening effect. It can dry out your hair and potentially make it more prone to breakage.
In short, developer needs to be mixed with bleach or high-lift color to work.