What did the Rebels intend as the role for their capital ships in the Battle of Endor?

Question

The epic Battle of Endor featured the entire Rebel fleet of capital ships. Six or more Mon Calamari cruisers (two were lost, and four more can be seen when Executor goes down), a few Nebulon-B frigates, even more CR90 Corvettes and GR-75 Medium Transports.

However, what puzzles me is: why were they brought there? The whole plan was for the fighter group to fly into the superstructure and blow up the reactor.

So, what was the role of the capital ships? Since the goal is to blow up the reactor anyway, bombarding the station serves little purpose, especially since it might have had operational turbolasers, presenting a threat to the ships.

Ackbar mentions that:

(...) our cruisers will create a perimeter while the fighters (...)

What does that entail? Surely, not engaging the supposedly absent Imperial Fleet. Intercepting potential evacuees? Because, a wing of A-Wings would perform infinitely better than an MC80 cruiser.

Answer 1

There are several reasons why the Rebels would deploy capital ships:

1. The Rebel capital ships can fight Imperial capital ships. This can be useful for hindering the Empire's ability to deploy TIE fighters against Rebel fighters. Note that the Rebels were unaware that much of the Imperial Fleet was hiding on the far side of the forest moon, but they had to have expected at least a few Imperial capital ships. After all, the Empire would surely deploy some Star Destroyers with the Emperor on board (especially if the Death Star II was not operational, as the Rebels thought). Also, the first Death Star was protected by Star Destroyers during construction so the Rebels would expect similar protection for the second Death Star:

2. The Rebels deployed medical frigates, such as Redemption, and some other non-combat ships like transports. The medical frigate could tend to the wounded (e.g. Rebel troops returning from the forest moon), and the transports were probably necessary to bring equipment (such as fuel). Rebel capital ships are necessary to protect these non-combat ships from Imperial Star Destroyers and/or the Death Star itself.
3. The Rebel capital ships can also carry supplies for the Rebel fighters, and can act as carriers to repair/refuel/re-arm Rebel fighters during the battle.
4. Rebel capital ships could be used for orbital bombardment of Imperial units on the forest moon of Endor, in support of the Rebel troops attacking the shield generator (obviously only after the shield was down).
5. Although likely not very effective against fast TIE fighters and interceptors, the capital ships might be able to score a few kills against Imperial fighters anyway.
6. The Rebel capital ships could attempt to prevent important Imperial officials (such as the Emperor himself) from escaping before the Death Star was destroyed. This is likely the reason why Admiral Ackbar said the capital ships would form a "perimeter".
7. The Rebels knew the Battle of Endor would be decisive: either the Empire would suffer a terrible blow with the loss of the Emperor and the second Death Star, or the Rebels would fail and the rebellion would be ruined. Why not send everything you've got, just in case?

Answer 2

The Star Wars in 100 Scenes factbook describes the Rebel's capital ships as acting as fighter-carriers.

They plan for a commando team to land on the moon and bring down the shield before the rebel fleet arrives and launches starfighters to fly into the Death Star and hit its main reactor.

Interestingly the film's official novelisation strongly suggests that the main fleet was also going to directly assault the Death Star II.

The Rebel fleet broke out of hyperspace with an awesome roar. Amid glistening streamers of light, battalion after battalion emerged in formation, to fire off toward the Death Star and its Sanctuary Moon hovering brightly in the close distance. Soon the entire navy was bearing down on its target, the Millennium Falcon in the lead.

_{^{Star Wars: The Return of the Jedi - Official Novelisation}}

Answer 3

The larger rebel ships still have guns that can shoot down enemy fighters. What's better? sending a bunch of fighters at the death star, or a bunch of fighters with additional support?