艹 is the top-position form of 草 (grass) and signals plant material. It narrows 新 from generic newness to specifically chopped vegetation - firewood - and groups 薪 with other gathered-plant chars like 茶, 药, 菜.
新 gives the sound xīn directly and preserves a faint semantic echo: 新 itself originally meant freshly cut firewood (note the 斤 axe on its right). 薪 keeps that older sense alive once 新 drifted to mean new.
