Uncle Ob's whirligigs aren't just weird metal sculpture doodads—nope, they represent his creativity and his most personal self, too. When Uncle Ob was with Aunt May, he was able to make all these whirligigs because Aunt May supported him and made him feel like he was amazing just the way he was. Aunt May always bragged about Uncle Ob and made him feel like what he was doing was important and wonderful.
Ob was never embarrassed about being a disabled navy man who fiddled with whirligigs all day long, and I never was embarrassed about being a kid who'd been passed around for years. We had May to brag on us both. And we felt strong. (2.40)
When Aunt May dies though, Uncle Ob ends up putting the whirligigs aside—it's as though he can no longer create art now that his support system is gone. However, as the book goes on, Uncle Ob starts to realize that Aunt May is still with them, and that she'd want them to be happy and to continue doing what's important to them. That's why at the end he puts up all the whirligigs in Aunt May's garden—he's using his art to honor her, since she always believed in him so much.