Can change level which is good for Synchro summoning.
Copying the level of another plant monster means that you can only Synchro summon even leveled monsters with Copy Plant and the monster he copied.
Use Copy Plant to Synchro summon the turn you summon him because it is unlikely he will survive your opponent's turn.
Since Copy Plant can copy the level of other Plant monsters, you can use it alongside other cards that can help you access Synchro summons quicker and easier. Homunculus the Alchemic Being can change his own attribute allowing you to access Synchro monsters that require specific attribute monsters. Lekunga can special summon Tokens you can use to Synchro summon. World Carrotweight Champion can special summon himself from the graveyard to use alongside Copy Plant as Synchro material.
This ability has no sense: right now it is the only way to have Black Rose Dragon in your side deck but it also adds to your deck 2 useless tuner monsters whose effect can only be used to synchro summon 6-8-10 level monsters. Or you just have to hope your opponent uses lv 1-3 or 5 plant monsters to copy that and summon BR D. Either way I see no utility to use Copy Plant with Black Rose Dragon.
You can search for it with Super Solar Nutrient. This gives Aromage decks access to Stardust Dragon i.e. Super Solar in Rose Lover, special summon Rosemary, use copy plant with Rosemary. There's plenty of other combos that work too.