The Makings of a Good Invasion: Motive
Invasions, especially in Traveller must be gosh darned expensive (sorry for the language. Each soldier transported will require part of a stateroom and cost Cr 8,000 or more. Each ton of vehicles, supplies and gear will cost Cr 1,000 and you will need a lot of such cargo. thought I am pretty sure that a government operation will use transports owned outright such ships still have maintenance life support and crew salaries/ They might even have mortgages. You get the idea. Money money money.
Now, even invasions need to be cost effective. There has to be something the invaders want and are very likely to get. In the OTU, two cultures have themselves set up for constant invading, the K'kree and the Aslan. The Aslan want land because status goes with it. Cost is secondary (the females will handle it). The K'kree want to pacify carnivores (their definition of 'pacify' is expanded to include exterminate.) In both these cases it's an ideology requiring expansion and conquest. Money is no object -provided you have the money.
There are political objectives. If a planet is supporting pirates attacking your ships, you might have to invade Tham and blitz the pirates. The same holds true for any planets supporting group out to hurt you. On the darker side of things, you might need to occupy a planet looking to secede from your polity, to show the others what could happen. Again this might overrule the cost.
Resources are a popular reason in the movies and comics. Most of the time the resources are laughable. Why go after water? Any Oort Cloud has countless ice balls to plunder. Slaves? Automation is a lot cheaper and less likely to slit your throat. Raw materials are as likely to be in the invaders' system as any place they invade unless there are really exotic forms of unobtainium to be had.
Invaders may be after living room. Garden planets are not that common. But there are a lot of middling planets that you could make more comfortable. Space colonies are another option with plenty of other benefits (manufacturing, farming, power generation).
However people seem to want planets very badly.
Organic compounds might be worth traveling a few parsecs for. Your planet of vampires needs human blood! Less dramatically, the planet's eco system provides ingredients for important pharmaceuticals. Perhaps you're a psi hater and local plants can produce psi booster. Perhaps they won't share their anagathic trees with you.
Finally, maybe the invaders are deluded and seeking a treasure that does not exist. Remember the conquistadors seeking the Fountain of Youth and the Seven Cities of Gold?