用不着 is colloquial and slightly dismissive — it implies 'don't bother.' The positive form 用得着 means 'might need' and often appears in rhetorical questions: 用得着这么生气吗?(Is there really a need to be so angry?)
grammar
This is a potential complement structure: 用 + 不 + 着. The 不 indicates inability to reach the result (着 = achieving contact/use).