They didn't need Loki, they needed Dr Erik Selvig: or to be totally accurate, they needed someone to open the gate for them (or in the case of the Avengers, an orchestrated attempt to use the power of the Scepter to create the second half of a 'bridge' for the dimensional vortex to work)
Loki was their agent, and he used his cunning and craft to manipulate the relevant factors, and find elements that could integrate the power of the Tesseract with the Mind Stone within Loki's staff to achieve just such an event.
They didn't need Loki, they needed a way onto earth. Loki promised to provide it to them, should they grant him possession of the Scepter.
it can be used to "activate" the Tesseract, the Scepter played no part in activating the Tesseract. It was used to pierce the shield protecting it though.