Mikko wrote:

Why is that speed needed?

So you don't hit the ground when gravity pulls you down.

Orbital velocity for a given altitude could be defined as, "Moving fast
enough so that when gravity pulls you down far enough to hit the
ground, you've moved far enough sideways that you miss the horizon."

When you're at an altitude of 200-300 miles (like the shuttle or
International Space Station), you only have minutes to scoot sideways
far enough to miss the horizon. Earth's gravity is only barely
diminished at an altitude of 200-300 miles, so you're falling quickly.

For example, it only takes 22-23 minutes for the shuttle or ISS to fall
far enough to hit the core of the Earth. The shuttle and ISS need to
get about 4000 miles sideways in 22-23 minutes to miss the core,
mantle, crust, and atmosphere, or they're going to leave a pile of
metallic confetti on the ground. That calls for a high velocity. In
fact, the magical number is 17500mph.

But also gravity gets smaller when you get more away from the earth?
How high does one have to go to have only half of gravity?

About 1650 miles, deep in the inner Van Allen radiation belt.

What if someone built a 30,000 ft high tube, similar to magnetic trains
- electrical magnets around it. Then you could just put metallic
cargo inside - without any engine or fuel, and shoot it up.

And then it would fall back down and land near the launch point.

Well, unless you fired it at escape velocity, in which case the cargo
would just drift away from Earth, never to return. Most commercial
launchers like to have their satellites stay near Earth, where the
customers are.

Mike Miller