If you insert the same screw block 15 times in a drawing, AutoCAD LT stores the detailed block definition only once. The 15 block inserts that point to the block definition are very compact and take up much less disk space than 15 copies of all the lines, polylines, and arcs. The ability to edit all instances of a symbol in a drawing simply by modifying a single block definition. For instance, if your design requires a different kind of screw, you simply redefine the screw's block definition. AutoCAD LT then replaces all 15 screws automatically with this new recipe.
Blocks are great for convenience and saving storage space within a drawing.
Blocks aren't as great for drawing elements used in multiple drawings, however, especially in a multiuser environment. That's because later modifications to a block definition in one drawing do not automatically modify all the other drawings that use that block. So if you use a block with your company's logo in a number of drawings and then decide to change the logo, you must make the change within each drawing that uses the block. Then select some objects, choose the Create Group button, and type a name for the group. If you've toggled it off, picking an object selects only that object, even if it happens to be a member of a group.