Why do you need to shoot your own sonic tanks?

In this Dune 2 speedrun, the player builds sonic tanks and sends them to destroy the enemy base. Before sending each tank, he exposes it to friendly fire until it gets a certain amount of damage. He even has a dedicated regular tank whose sole "mission" is to shoot own sonic tanks before sending them to battle.

What benefit does this give?

Below is a link to the video with a timestamp. He explains this earlier in the video, but the sound quality is so bad it's completely unintelligible.

Link

It appears he is utilizing this glitch in the game:

Invincible

Required: Atreides, Level 5 or higher, High Tech Facility, Repair Facility.

Build walls around the Repair Facility to block the entering passage, and Build a Carryall with the High Tech Factory. then damage one tank or any vehicle until the damage meter gets yellow, the Carryall transport it to the repair facility, and automaticly put it back. now the tank is invincible, when your unit approach the enemy base, they didn't notice you, by any chase destroy all buildings. only a unit will attack you if you attack him.

NOTE: Invincible is only able with Atreides and level 5 or higher. | Submitted by DennisDreessen

This works well with sonic tanks as they don't actually target anything when they attack, as their shots can do damage even when targeting terrain. If you notice when he gives the attack order on his tanks he's actually attacking terrain instead of a building or a unit.

Source