Why did Grindelwald organise this event in the first place?

Question

In Crimes of Grindelwald we see the title character

organize his escape

during his transportation from MACUSA prison to Europe.

However, we soon find out that

all this time, he was disguised as Abernathy,

who later Apparates to the carriage transporting the prisoner and a few wizards. At that point he is already in possession of the Elder Wand and the Bloodpact, as Abernathy is seen giving those two objects to Spielman, before the carriage leaves.

ABERNATHY approaches them.

ABERNATHY

"Mr. Spielman, we found his wand hidden away."

He hands over a black rectangular box.

PICQUERY

"Abernathy?"

ABERNATHY

"And we found this."

He holds a vial of some glowing gold substance in the palm of his hand. SPIELMAN reaches for the vial, which hangs on a chain, and after a moment of hesitation, ABERNATHY releases it.

^{The Crimes of Grindelwald - The Original Screenplay - Scene 5}

If Grindelwald had the Elder Wand and the Bloodpact and he was in fact free, why did he bother plan "his" escape and didn't just flee?

Answer 1

In order to escape prison, Grindelwald and his acolyte Abernathy switch places. Grindelwald could have run off with the vial, allowing his replacement to rot in jail. But Grindelwald's loyal followers are not yet so numerous that he can afford to waste them. To do so might send the wrong message to other followers.

When Madame Picquery reminds "Abernathy" to hand the vial over to Spielman, the viewer can see that he is hesitant to do so- just as the script says. To get the vial back, "Abernathy" has to keep up appearances until the right moment. So he waits until the carriage is in transit.

Grindelwald now has two reasons to attack the carriage. First, to retrieve the vial which is no longer in his possession, and second, to collect a useful and loyal servant.

Answer 2

He may have wanted his escape known.

Escaping while Abernathy took his place in the prison cell would certainly be more effective in allowing Grindelwald to escape unnoticed. However, Grindelwald may have actually wanted it known that he had escaped. After throwing Spielman out of the carriage, he threw a wand after him, giving Spielman a chance to save himself. If Grindelwald did not want word spread of his escape, it would be better to simply kill him instead of leaving behind a possible witness.

“GRINDELWALD
So needy.

He then flings it through the door.

He blasts SPIELMAN magically through the open door, then tosses a wand after him.

SCENE 16
EXT. SKY OVER ATLANTIC OCEAN—NIGHT

As SPIELMAN falls, he manages to seize the wand and conjures an invisible Slowing Charm. Sinking slowly toward the sea, SPIELMAN watches his carriage streaking away in the direction of Europe.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

From the dramatic manner of his escape and his giving a witness the chance to live, it seems more likely that Grindelwald wanted word of his escape to get out, as evidence of his power. When he was captured, he implies that it would be impossible for wizarding law enforcement to keep him captive.

“MADAM PICQUERY moves towards him.

GRINDELWALD
(with contempt)
Do you think you can hold me?”
_{^{- Fantastic Beasts and Where to Find Them: The Original Screenplay}}

Grindelwald likely wanted it known that he had escaped, and that MACUSA was indeed unable to hold him. He probably wanted it known that a wizard of his caliber would not be easily kept in custody by the wizarding authorities.

Also, loyal followers are not easy to find.

Though his probable desire to have his escape known was likely the biggest factor in his decision, Grindelwald would also likely be hesitant to waste a loyal follower by having him sit in a cell indefinitely. While many wizards were sympathetic to his cause, his inner circle of wizards willing to play a larger part in his plans seemed to be about seven at the time right after his escape.

“SCENE 23
EXT. ELEGANT STREET OF 19TH-CENTURY PARISIAN HOUSES—AFTERNOON

GRINDELWALD and ACOLYTES stand in the street. GRINDELWALD points his cane at a particularly fine house.

A clatter announces the arrival of a horse-drawn hearse.

NAGEL, KRALL, CARROW, ABERNATHY, KRAFFT, ROSIER (female), and MACDUFF approach the front door. KRALL opens it with his wand. The ACOLYTES enter.”
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

These (except Krall) were the ones truly loyal to him, and therefore the ones who would be the most willing to follow him around the world and complete whatever tasks he needed them to. Without Krall, he seemingly only had six of these truly loyal followers - making them a very valuable commodity.

“GRINDELWALD
Protego diabolica.

He spins and draws a protective circle of black fire around himself. The exits close.

ABERNATHY, CARROW, KRAFFT, MACDUFF, NAGEL, and ROSIER walk through the flames into the circle.

ANGLE ON KRALL, hesitating.

Then he decides the circle is the better option, braces himself, runs into the fire—and is consumed.”
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

Grindelwald would likely be reasonably unwilling to waste a loyal follower on something as useless as sitting in a jail cell, particularly when escaping another way that does not involve this also gains him more notoriety.