Spreading mulch in your garden helps protect the soil in general, but it has specific benefits during harsh winter conditions. A layer of mulch will keep the soil insulated, roots protected from quickly freezing or overly wet soil, and you’ll also end up with better results come spring by laying down mulch in the cold months. Image from ogier300.
Kris Wetherbee, writing at The Oregonian, explains the benefits in more detail:
Fall and winter mulches create an insulating barrier between the soil and air, thereby protecting plant roots from rapid fluctuations in soil temperature. The temperature changes create movement that can heave plants out of the soil, exposing their crowns or upper root systems to damaging freezes. Fall and winter mulches also protect the ground from soil erosion and compaction from heavy rains.
You can also use mulch in winter to line dirt paths that turn to mud during the wet season. Different types of mulch (shredded leaves, wood chips, straw) are best used in different areas of your garden, and the full guide linked below explains where to use which type of mulch.
What You Need to Know About Winter Mulch [Oregon Live]