I have used them when I wanted to move, copy, or rotate groups of sketch entities, that are not fully defined. Also I have used them to check linkage function before I get to the extrude stage.
The linkage thing that Jerry mentioned is definitely one of the primary uses. You are correct that you could create solids to represent these things. But, it is not twice the work. SW has a feature where you can reuse the same sketch and decide what profiles from that sketch to use. So, you can draw your entire mechanism in one sketch, check the motion of it in that sketch, and then use portions of that sketch to create each body of a mechanism.
Blocks are useful because you can save them and use them again later without needing to re-sketch the lines. So for example I have a block saved of our company logo that I can easily throw on a part and scale if we want to engrave the logo. Also as Jerry mentioned it is nice for moving around groups of lines. So when I am designing a decal and I want to move the icons around it is easy to just drag them around if they are each a block.
Thanks fellows, Jamil set off the ding ding ding, with the logo deal.
So sketch it on a napkin(pc) and keep in the draw(hd) for future use alright.
Can't give all three the correct answer but I still thank Jerry and Eric.