Why did HMS Unicorn need a flight deck?

Why did a forward aviation support ship need its own flight deck?

It allowed maintainers to test repaired aircraft and let serviceable or replacement aircraft fly directly back to operational carriers.

The flight deck was part of the maintenance system rather than decorative carrier mimicry. Repaired aircraft needed a practical way to prove they were airworthy before returning to an operational unit. Aircraft that worked, along with new replacements, could then fly directly to a carrier even when a boat transfer would be difficult in rough seas. Damaged aircraft still had to reach Unicorn by tender, so the deck made the outward half of the repair cycle much easier.