Napoleon wanted mainly two things from the Italian unification, which were essential for maintaining France standing and integrity on the continent.

First : decreasing austrian influence in eastern Europe and the italian peninsula. An united peninsula would be more capable to defend against the austrian pretentions in Northern Italy than half a dozen rival states with no budget or human means to properly defend against the might of the austrian army and the meddling power of austrian diplomats.

Second : the absolute french control of the western alps. The fact the Kingdom of Sardinia controled the western alpine passes was considered a huge liability by the french politics and military commands. These passes were considered critical for the defense of the French South Eastern border but were not controlled by the french military, and the alliance games of the time and the fact Sardinia wasn't considered to wisthand an austrian assault were nightmare fuel for the french military.

With the Plombières Agreement, Napoléon killed two birds with one stone : France would gain control of the western passes with the annexation of Savoie and Nice, and by supporting the sardinian military with troops and funds it would fulfill the first objective.