As Rich notes, Ajdabiyah is a fulcrum from which pro-Qaddafi forces could pivot to Benghazi and Tobruk — Libya’s two major port cities east of Tripoli — and controlling one or both could effectively choke off what is thought to be the rebels’ main supply routes from Egypt.
Tobruk, of course, is already a famous battlefield — the place where the Panzer blitzkrieg finally discovered how to work the brakes on their tanks. The Allies had lifted the port from the Italians in 1941, and took full advantage of the extensive perimeter defenses they found built there. With British resources — and crucially anti-air resources — diverted to Greece, Benghazi was a shooting gallery for Axis air power and unusable as a port of resupply, making Tobruk’s harbor the more vital. Later that year, when Irwin Rommel’s newly arrived Afrika Korps attacked the fortress, they were repelled by a ragtag force built around a single Australian division. Rommel laid siege for 240 days before the Australians were finally relieved by the newly constituted Eighth Army as part of Operation Crusader, at which point Rommel had bigger fish to fry.