DC-DC chargers generally (not addressing nor recommending that Redarc)
accomplish three related goals. None are just for alternators, and implemented properly lead to not only more precise (healthier for the LFP bank) but **faster** charge cycles.
First off, matching the desired charge profile of the target battery rather than accepting whatever the alternator+regulator puts out.
Really needs to be adjustable, not just a few canned choices but user-custom.
Properly regulated stages, dropping to lower Float voltage once the bank is Full (pertains more to lead than LFP)
This precision is not just safer, but results in much longer bank lifespan, obviously alone justifies the cost when spending thousands on LFP cells.
Second, compensating for voltage drop when the target bank is a long way from the charge source. DCDC goes at the bank, converts whatever widely varying input and outputs precise voltage desired.
Third, for LFP only, throttles the current pulled, prevents the bank overloading / overheating / burning out the alternator+regulator especially electronic components.
It is true that replacing the stock alternator + regulator can get the same results, converting to fancy regulators that can adjust power output, amps not just volts to match temperature conditions, override when all HP is required for propulsion etc
but usually that is not even an option for most modern production land vehicles.