I wrote a fair bit about how all out nifty ships, any ships are in many of not most planets in a Traveller universe. They can go darn near anywhere, produce gigawatts with a few tons of water etc.
They can bring goods from places you could never reach yourself. Heck staying in orbit a few weeks and monitoring weather might spell the difference between starvation and bountiful harvests on some worlds.
That's just if you take the high road. Tech 6 or lower worlds could do very little against a ship that hovers outside the atmosphere and extorts money by threatening to blow up monuments or government buildings. If you're playing with the '77 rule set then outside of your commercial lanes the odd isolated planet won't have law ships or the Navy come by very often. Why can't you take your war surplus laser cannon and take what you want? Why can't you be king?
A few things to consider:
If you thought about doing it someone else probably did and before you. Any planets with decent resources and industrial bases will be around for centuries. They lived this long and you didn't notice any laser burned buildings on the scanners so something must be up.
First if you can buy a jump tape to the planet (see Classic Traveller) odds are you are still too close to that Remote Centralized Government which will be come up close and personal pretty fast if you manage to hit the trigger buttons. For that matter the Close By Centralized Government of the starport that sold you the jump tape might take a dim view of a pack of murder hobos torching a sister world and possibly setting up a base for marauders. It's bad for business.
Second don't underestimate the power and reach of the Scout Service. Yes the Navy basically hangs around points of strategic interest but the scouts go everywhere. They have better ships than the dinky one they loaned you (how's that for irony), they have good sensors and they have weapons and can give a shout to the Navy for threats above their annual budget.
Third taking over a world will put you in the position of the Tyrannical Local Government. Look for s steady stream of adventurer types attempting to overthrow you. We all know how that ends up eventually.
Fourth the kind of world you are looking for is probably agricultural or has an economy based on gathering natural resources. That requires people. Shooting up the place or overthrowing the government is likely to disrupt this activity and the credits will dry up. People expecting those credits will start getting concerned.
That's just the obvious things that can and will go wrong (referees hate changing the government they statted up for a world.)
Before you try taking over a world you might want to find out about all the stuff that doesn't appear in the travel guides. Say you've set your eyes on a world that pays tribute to a pirate clan (governments pay tribute, people pay protection money)? Those pirates are going to be a little put out. Ditto the smugglers who have arrangements with the scanner operators you replace with your own guys. Let's say you've taken over a world, what could go wrong?
1- The planet is the site of a huge credit laundering operation by a large evil conglomerate. You have more wealth than you will ever live to spend (in more ways than one).
2- One of the locals who went abroad became a very successful mercenary who is very upset to hear about your actions.
3- The Company has had a deal with the planetary government for decades to provide catalyzer crystals quietly thus preventing a mining boom and having to deal with the insanity of dealing with prospectors, secondary mining outfits and inevitably the crime, theft and cost of dealing with the above.
4- Another outfit had the same idea. They just were a tad slower than you. Now they have all those troops and ships paid in part ... expecting the other part.
5- A cache of Forerunner artifacts, catalyzer crystals or other Richezz were discovered on planet. A free for all ensues till the Government shows up (this works great as a rumor the deposed government starts!)
6- The planet as a secret military base with advanced research far from the Remote Central Government. It is well defended but will probably keep a low profile unless nosy invaders show up or the deposed government threatens exposure.