Because you don't want to leave the steroid in your mouth, it will cause thrush.
Steroids can cause overgrowth of a fungal Candida infection commonly called thrush. Normally a person always has both good and bad bacteria in their mouth. Usually there is a balance between them which prevents overgrowth but the steroid residue feeds the fungus and causes overgrowth so rinsing the mouth helps to remove the drug residue that encourages this growth.
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