Why a 170-gun first rate would enter ordinary

Why would a hypothetical 170-gun, four-deck first rate be placed in ordinary besides its heavy crew requirement?

Maintenance would be the main reason, while active service would most plausibly be as a Mediterranean flagship.

Maintenance is the main reason suggested for placing the hypothetical 170-gun first rate in ordinary. When commissioned, such a large ship would probably serve as the Mediterranean flagship, where its presence could be useful.