The goal was simply to stop the vessel from completing its planned 1985 trip to Moruroa to protest/stop the testing of French nuclear weapons. The tests were seen as vital to French national security and the only way to remain independent of US or USSR influence.

The bombing of the Rainbow Warrior only sounds shocking if we do not take into account the full history of French interference of groups who might oppose their nuclear testing pattern. France was very concerned about the activities of Greenpeace- they had agents infiltrate the group. They had worked hard to mute any opposition to testing from within French Polynesia, manufacturing a case to exile autonomy/independence minded leader Pouvanaa a Oopa until he accepted nuclear testing.

The whole thing was bizarre; so I can see why you are still asking why even after reading/learning about it. The affair, which landed several French agents in a New Zealand jail for some time, ended up far more embarrassing for France than if they had just allowed Greenpeace to attempt to interfere with the testing. More protesters ended up coming than might have otherwise been inclined and Greenpeace sent a greater part of their own resources attempting to disrupt the French testing after the sinking of the Rainbow Warrior in harbor.

Fyi, for those who don't know about this event: Metaphorically, one of the biggest bombs France detonated in the Pacific was the bomb secretly attached to the Greenpeace ship Rainbow Warrior. French agents set several explosives on the ship in an effort to keep it from interfering with upcoming French nuclear tests at Moruroa. The bombing was scandalous, and immediately suspicious. While terrorism is not new, nor new to the Pacific; the bombing of a peace ship in neutral and nuclear-free New Zealand was startling. French agents were sent on a bizarrely complex mission, posing as tourist divers and swiss holiday seekers in order to penetrate New Zealand. Their French accents and spending habits gave them away when locals searched for culprits after the bombing. The agents were careless, not hiding their activities very well and leaving behind a trail of evidence afterwards that made the whole venture painfully obvious; though hard to believe that France would bomb a peace ship in the harbor of an ally.

This was not the only crazy political move France made in the 80's- so like I said, we shouldn't see it in a vacuum. Between 1975-80, France was responsible for instigating secessionist movements against the new government in Vanuatu. In 1986-87 France was condemned by the South Pacific Forum and the UN for its actions in New Caledonia hostage crisis- hostages had been taken by Kanak independence activists, French special forces stormed the cave on Ouvéa where they were being held resulting in 20 deaths- the whole thing was pretty brutal with serious allegations that France had executed some of the hostage takers on the spot (the whole affair is part of a longer cycle of government violence in New Caledonia). In 1987 France offered aid to Fiji immediately after a coup when New Zealand and Australia had withdrawn foreign aid- French support for a military coup was not seen very favorable by other Pacific powers. The bombing of the Rainbow Warrior was just one in a long line of political missteps that damaged French influence in the region in the ensuing decades.

In addition to the other posts, the Rainbow Worrier would have acted as a "Mother ship" to many of the smaller yachts so allowing them to stay on site for longer.