I noticed that while the original intent was an orbital bombardment the Empire just ends up with a ground assault. Why even bother with the generator?
The IMPS even go into the base that was supposedly to be orbitally bombarded
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This boils down to Vader actually being quite a nice guy (shock horror!) or at the very least an effective leader who cared about his troops.
The original intention was a "clean" orbital bombardment of the base, something that would have resulted in zero Imperial casualties. When he found that the shields were already raised and the rebels were evidently planning an evacuation, Vader seems to have decided to go for more aggressive plan.
“He’s as clumsy as he is stupid,” Vader cut in, breathing heavily. “A clean bombardment is impossible through their energy field. Prepare your troops for a surface attack.” - ESB Novelisation
After (ahem) indicating his displeasure to Admiral Ozzel, Vader ordered the fleet be moved to create a blockade in space and he sent Veers to initiate a ground assault. The walkers would deposit a small number of ground troops and destroy the generators, followed by a swift aerial invasion of the Rebel base:
Through the cockpit window, Veers and his pilot can see the Rebel power generators in the distance.
A hologram of Darth Vader appears on a control panel screen.
VEERS : Yes, Lord Vader. I've reached the main power generator. The shield will be down in moments. You may start your landing. - ESB Script
Once the generators were down, the Empire was obviously able to land troops, including Vader's shuttle right on top of the Rebel base. This answers the immediate question of why the generators were such a choice target, not so they could start the (now largely irrelevant) orbital bombardment but so they could shuttle shock-troops onto the planet's surface.