someone remind me why the dowager queen needs him as a king?

She doesn't have a son, her husband the late emperor doesn't have an heir, she is essentially regarded as useless and will be refined to the palace/become a nun . Traditionally, the next in line will be the emperor's brother or their heirs, which is what the ml is, he's elligible for the throne and the jade pendant proves his legitimacy. However, the empress's power will still be diminished if ml's parents move into the palace and she will still be sent away, since ml's father will replace her as someone to provide royal guidance, but by killing them, she remains Empress dowager without someone of her generation standing in her way, she can rule the palace with essentially a puppet emperor. She sees him as someone easy to control since she cant do it on her own (she's a woman and she's not royal by bloodline). I'm assuming if ml does have a kid (regardless of who bears it), she will kill the ml and raise his kid with her acting as regent

ooo, thank you for explaining yeah that makes sense now, i just thought it would be really easy for him to get rid of her but i guess maybe not