I have a deck that has seen better days. It is a bit grey in some areas and fine in other areas. It is grey/weathered mostly where water pools or is exposed to the elements (i.e. not sheltered by the eaves).
So I assume given the inconsistency, sanding is essential?
Welcome to the Bunnings Workshop community. It's a pleasure to have you join us, and thank you for sharing your question about sanding your deck.
Sanding is essential if you are going to repaint or oil your deck. The old layers must be removed from the top of the timber so that the new paint or oil will be absorbed correctly into the timber. Poor preparation will result in the paint or oil peeling or coming off. I suggest having a quick look at this guide - How to restore a deck by @Adam_W.