Darth Vader tested carbonite-freezing technology on Han Solo because he didn't want to kill Luke who was precious to the emperor. But, why was it necessary? Wasn't it already a tested and proven technology? Even Darth Vader (when he was Anakin) himself used it along with others decades ago (Star Wars: The Clone Wars S03E18).
In the scene referred to in the question, the commander, Obi-Wan and Anakin himself all talk about this idea as though it is unproven and being completely improvised.
So while Vader knew it could be done, the chamber device and chamber probably still had to be calibrated and adjusted to the correct settings so that it would be done correctly.
No one says that it hasn't been done before. Vader's only uncertainty is in how powerful the freezing unit is.
Lando cautiously asserts,
"We only use this facility for carbon freezing. If you put him [Luke] in there, it might kill him."
Vader responds with,
"I don't wish for the Emperor's prize to be damaged. We'll test it first."
I am using the dialogue as it appears in the official Empire Strikes Back novelization.
The novelization gives an insight into Vader's thinking:
He knew a way to find out just how powerful this freezing unit was.
This test, of course, involves Han:
"Bring in Captain Solo."
The fact that Vader wants this done (freezing Luke in carbon as a means of transportation) could be inspired by his experiences in the Clone Wars. The novelization suggests that, as far as Vader is concerned, the only uncertainty is in how powerful the unit is, which is why Han is subjected to the procedure.
During the actual scene where Vader talks about freezing Luke, Lando expresses concern:
Lando: Lord Vader, we only use this facility for carbon freezing. If you put him in there it might kill him.
Darth Vader: I do not want the Emperor's prize damaged. We will test it on Captain Solo.
This leads one to believe that this is still a fairly experimental technology when it comes to freezing humanoids. But, in answer to your question, the reason for testing it on Han was to make sure he lived. In theory, if he lived, so would Luke.
There are many ways of storing and transporting cargo that are not necessarily suited for living beings. Would you like to be stuck in an airplane's baggage hold instead of its cabin?
The freezing process was most likely NEVER used for living beings, even though technically it was possible. It was only used for preserving perishable items for shipping. Naturally, using it on living beings would cause some trepidation.
I also felt there was some conflict between the tension in episode V (about whether or not Solo would survive the freezing process) and the generous use of carbon freezing in some of the newer productions.
But this isn't an outright contradiction if you look closely at this conversation between Darth Vader and Lando:
Vader: "This FACILITY IS CRUDE but it should be enough to freeze Skywalker." Lando: "We only use this for carbon freezing. You put him in there it could kill him!" Vader: "I do not want the Emperor's prize damaged. We will test it on Captain Solo."
So Vader's concern is the "crudeness" of the facility. But one could argue that the ease of carbon freezing in other productions undermines the suspense of episode V. We are supposed to be worried that Han Solo might die, right? In a universe where people are being frozen all the time, we are not so worried about Han (but that's just my opinion).
*Alternate/ additional interpretation: Lando is trying to thwart Vader's efforts by coming up with excuses, but he knows the freezing will probably work... and Vader also knows the freezing will probably work but he is being overprotective toward his son, because subconsciously he cares about him.