Calling a particle zāʾid (additional) is a grammatical description. It can mean that the underlying proposition could be constructed without the particle, while the expressed particle still contributes emphasis or another rhetorical effect.
Under the analysis that treats the kāf in “laysa ka-mithlihi shayʾ” as additional, the negation of likeness is forcefully expressed. Dismissing the particle as meaningless would obscure the rhetorical value claimed in this analysis.
Grammatical addition should therefore not be confused with pointless padding. Grammar explains how the sentence is formed; rhetoric considers why the chosen form can be more forceful.