Comment by philwelch

You probably wouldn’t use a ship like this in an initial landing, but once you’d secured a beachhead you could easily use it to deliver reinforcements and supplies. A major part of the value of a RORO ship is that you need little to no port infrastructure to unload it. Once you’ve secured a beachhead and built some minimal infrastructure on it you could land tanks and other armored vehicles, or you could also land trucks filled with fuel, ammunition, supplies, infantry, and so forth. This is, in practice, most of the work that LST’s did as well.

And it’s very common to requisition merchant RORO ships for amphibious operations. The British did so during the Falklands War (https://en.m.wikipedia.org/wiki/SS_Atlantic_Conveyor).