Deleting things is scary: “What if I need it later?” Fear causes pruning paralysis, clutter, and hoarding.
The trash can metaphor is so useful because it removes the two fears of “I might need that” and “oops my finger slipped.” The trash can is really a safety net, and it strongly affects users’ feelings of control and security.
That’s why it’s so frustrating that you almost never see trash cans in web applications. Instead we get lazy confirmation dialogs which only add friction and stress.
While they’re at it, web apps need to make bulk deleting much easier, (ever had to click a million checkboxes so you can “delete all selected”?) but that’s a whole other blog post.