There is never nothing to gain.
Life is one thing you can gain. Earth being completely 100% frozen for thousands of years and the land formations on LV-426 are not so different. I am confident that no matter what the aliens throw at earth life will always continue until the earth itself is blown to tiny pieces.
There are probably still many mysteries on earth that we probably would be dying to discover, where is the nearest planet similar to earth?
Now that I think about it it is like a more large scale alien version of Independence Day. "We will not go quietly into the night"! It also came to mind, earth is important because it has something in large quantities that the rest of the planets around don't have and that's WATER. Can't live on other planets without that can you.
Also for the invasion itself its not like I am just dropping thousands of marines on earth. There will be days or weeks of bombardments from orbiting ships across the atmosphere and precision air strikes. Then the marines go in and do their job as well as using tactics that defeat their tactics. Especially with the use of flamethrowers. The landing zones would have to be few though. Two or three in each continent so that the human armies would not be spread thin right at the start. Yes the war would be long but we need to retake the earth in baby steps otherwise the humans have already lost. And of course the isn't just shooting and dodging, the primary mission is to infiltrate hives, eliminate queens and eggs, then destroy hives with explosives. Elimination of all other aliens is secondary but not mandatory.
Changes I might make to it such as the timeline. I don't know if I want to get rid of my female human warrior in the predator clan, I like that so much!