The reason why I think the power of the senate also has to be involved in the removal of a person that was appointed by the president is in order to ensure that the person that is being removed is out for the right reasons and also go through the time that they have spent in office.
The senate of the nation would have to ensure that all of the people that the president has nominated have gone through a session of scrutiny in order to assess the person well enough.
In this case, while the person is to be removed, the senate should also be able to assess the time that the person spent in office and the work they did. This would ensure that the person is able to pass off as a good appointee during their time in office.
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