The Diplomacy rules state that
In some rare cases, orders are written so that an Army could arrive at its destination either by land or convoy. When this happens, the following qualifiers apply:
If at least one of the convoying Fleets belongs to the player who controls the Army, then the convoy is used. The land route is disregarded.
If none of the convoying Fleets belongs to the player who controls the Army, then the land route is used. However, the player controlling the Army can use the convoy route if he or she indicated “via convoy” on the Army move order in question.
This prevents foreign powers from kidnapping an Army and convoying it against its will.
How would moving by convoy (regardless of whose Fleet is convoying) ever effect gameplay? How would it be any different from moving by land?