They are actually quite malleable, which is kind of required because the contact area could be imperfect or ever so slightly angled.
A bolt on the other hand, there is almost no malleability there at all. Most bolts are rigid and inflexible because of their geometry and because the tiny threads need to be very strong.
In recent years, washers have evolved a little further and now many washers also include a locking feature to prevent the bolt from coming undone. You also see thicker washers and spring washers which are designed to regulate the bolt force even if the temperature varies a lot and the bolt expands or contracts.
Finally, the washer material might be important. Steel bolts corrode against everything, so most washers that you find are probably zinc plated or similar and designed to insulate the bolt head material from the contact surface material.
It can spread out a load on a hole, including if the holes are made oversized to make up for tolerance variations in the parts. This is useful for bolting together wooden structures, or softer materials like aluminum.
Stocking a variety of washers could be cheaper than stocking multiple types of bolt heads and nuts.
It can be a spacer.
There are specialized washers that can take up small angular tolerance variations, such as the spherical washers in some bike brake pads.
You don't see them everywhere, and using them by habit isn't necessarily the right thing to do in all cases.