The Spectrum app allows you to directly access the channels included in your Spectrum account via the Roku, essentially bypassing the Spectrum cable box. If you have no Spectrum cable subscription you have no use for the Spectrum app.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X