Loki didn’t want to destroy Earth, he wanted to rule it. He’s only attacking New York, where the Avengers are located, not the entire planet.
The Chitauri didn’t agree to help Loki, the Chitauri was part of Thanos’ army which he sent to help Loki, and in return for ruling Earth he’d give Thanos the tesseract.
Loki wants to rule Earth because like all Asgardians he views Earth as inferior, the weakest of the Nine Realms which makes Earth easy for him to conquest and rule. You could even argue he was doing it to spite Thor who loves Midguard. I mean all this is in the movie. Years ago some of us in here even believed that this was all part of Loki’s plan in order to get back to Asgard and get his revenge, which actually worked out as he was King of Asgard for a while and even banished Odin himself.
Thor 1 and Avengers he was a huge threat. Thor 2 he was imprisoned by Odin and “died” by the guy who was giving Thor them 808s and heartbreaks. I agree, he didn’t need to be in Ragnarok but he’s been the bright spot of all the Thor films so naturally he was put in it.