Death and taxes are mandatory. Gravity is mandatory.

In many/most situations direct-charging the house bank with a combiner (relay, isolator, VSR) is both effective and inexpensive. Even with lithium banks.

There are scenarios where one may want to disable the combiner.

• When the vehicle has a smart (variable-voltage) alternator
  • Common on European vehicles, and on American vehicles newer than ~2017
• Small alternators vs. ginormous battery banks of any chemistry
• Exotic alternator or battery bank voltages, including setups like 24v house bank and 12v alternator (or vice versa)
• If one needs relatively-stable charge rates¹⁾
• If you want to impose an arbitrary current limit²⁾
• when an influencer says so
• when Chicken Little claims you need one or the sky will fall

• lead house bank when alternator is the only charging source

In this case DC-DC won't help with Absorption duration³⁾ but it can help with Absorption voltage.