There are two reasons to convert a block to a group:
1) Individual objects in a group can be edited and modified while individual objects in a block cannot.
2) When editing a block, converting it to a group makes it easier to convert back to a block after it is edited.
When you convert a block to a group you are, in essence, adding a local copy of the block to the drawing. You can now modify the group as needed without affecting any objects that reference the original block.
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Editing a Block Reference
Using Blocks from Other CAD Programs
Storing Blocks in the Drawing
Grouping Drawing Objects
No CAD Zone Drawings
Using Drawing View
Smart Drawing Objects
Working with Drawing Objects