system
Given the following post, respond as if you're commenting on Reddit:
human
[Thunderbirds] Why do they exist?

These guys seem to serve no legitimate function. If they are truly a rescue operation, how do they respond to real disasters? A jet, submarine and rocket don't exactly stack up against large scale landslides burying thousands in rural communities. 

And how is their tech so advanced anyway? Hover bikes? Space travel? Aren't they just a small wealthy family?
gpt
Firstly remember that in the 2060s International Rescue's technology isn't that much more advanced than any other - just look at the Fire Flash or Sun Probe. Hover Bikes and Space Travel are commonplace - you may have heard of the time they rescued the crew of a pirate radio station that not only had their own satellite, but broadcast directly from it, with the hosts actually living up there. 

Also remember that the Tracies are more than just a small wealthy family, they're billionaires with agents all over the world, some of whom, like Lady Penelope, certainly contribute. Once a machine is designed, they have components manufactured all over the world from different companies. 

How do they design them? Brains is a genius, and Jeff is an astronaut who went to the moon - he also knows a lot about design of this type of machine. 

Thirdly, the Thunderbirds themselves are far more than a "jet, submarine and rocket".

* Thunderbird 1 is a VTOL rocket/plane hybrid capable of flying at Mach 20 used to recon disaster sites - it's capable of reaching anywhere on Earth in under an hour and a half. 

* Thunderbird 2 is a heavy equipment transport ship capable of flying at Mach 7 used to transport IR's main rescue machinery to the disaster zone. This is where you underestimate them most, by forgetting about this aspect - the Thunderbirds themselves are mostly just transport, its the pod vehicles that really do the rescuing usually. They have at least tens of different specialized machines.

* Thunderbird 3 is their rocket, mostly used to reach Thunderbird 5 but also for any space rescues that may occur. Again, not a true rescue vehicle but still essential.

* Thunderbird 4, you're right. It shouldn't be a main Thunderbird, just a pod vehicle, since its uses are highly specialized. 

* Thunderbird 5 is their satellite used to monitor for distress calls and coordinate rescue missions. Its highly important to IR's operation.

Finally, how do they respond to real, big disasters? They don't. Even with a Mach 20 rocket, International rescue are rarely the first responders to a disaster. Thousands might die in a disaster, but no one can prevent that, and standard rescue organizations are more than capable of dealing with the aftermath, 3 or 4 guys from IR don't make much difference to that.

A common situation International Rescue deal with is where people are trapped, but alive, but trapped where no one can reach them or save them. This is where their specialist machinery is useful. 

Rescuing survivors after an Earthquake is a job for the local organizations and requires hundreds of people. Rescuing people trapped in a burning building, badly damaged by the Earthquake which will collapse and kill them if any normal rescue operations take place, that's when you call International Rescue. 

They also deal with much more outside the box situations, such as terrorist attacks. When the Fire Flash was trapped in the sky because of bombs attached to its landing gear, only International Rescue had equipment that allowed them to pull off a landing without disturbing the bombs, which saved the lives of everyone on board and prevented a highly dangerous crash of a nuclear powered plane. 

So all in all, they have a clear purpose and situations which they deal with. They aren't a few guys who go out to help in every disaster relief operation around the world, they're a few guys who come to save people that no one else can save.