I don't understand why batterieds are needed if they are feed from an EM source? Aren't batteries required where the lights are feed from a normal source so they stay lit?
Because it does not appear you have an 'EM' source, you have an 'Optional Standby' source and optional standby sources do not qualify to supply emergency lighting.
There is a lot to this and for sake of keep it short I will generalize.
An
Emergency Generator as covered by Article 700 can only supply loads that the AHJ determines are true
emergency loads, like egress lighting. It cannot (normally) supply HVAC units, coffee makers or even the telephone equipment.
All the wiring between the load and the emergency generator must be kept entirely separate from other wiring. No sharing raceways, panels, transfer switches, junction boxes (there are some exceptions) This adds a lot of work and equipment. The emergency circuits are treated like they are sacred.
On the other hand
An
Optional standby generator can supply whatever loads the designer wants
except emergency loads. An optional standby generator
cannot directly supply emergency lighting.
With optional systems the wiring can be mixed in the same conduits and raceways as other wiring.
So you mentioned that this circuit starts at a panel that supplies other loads like HVAC units. This very strongly suggests you are dealing with an optional standby generator and that being the case it cannot be used to directly supply emergency lighting.
In this case the battery units happen to take care of the emergency lighting requirements, but the designer also decided to take advantage of the generator on site. Now during a power failure the battery takes over and if the standby generator does run as it should it will power up the fixture and recharge the battery unit. However if the standby fails you will still have the required emergcny lighting.