They're good for when you want to stop something on a production line while you drop something else onto it to get welded.
Also keep in mind that you can attach other blocks TO pushers and blockers, so you can have stuff like this:
You can get some pretty creative solutions going by moving other blocks around with pushers. You can also move multiple blocks with a single pusher, as long as they're not attached to anything else when the level starts.
Also, as you might have noticed, any block that isn't attached to anything else when the level starts will just fall. You can use this to set up some more sophisticated automation if you want. A loose block moving around on its own on a conveyor belt will still set off sensors, just like the production blocks do, like so:
Now obviously, that's a pretty useless application, but it's just an example. It's great if you want to have several things happen in a row, but with a specific delay between them. You just wire sensors along the conveyor track at the right distances apart, and let the loose block do your timing for you.
