Not a CS person, but I think if you keep the type information pined to the object in memory then a debugger can use the object type information in memory to match it with the object's definition in the program. That makes creating a debugger easy.

Imagine wanting to know what the object at address 0x34199920 is. If you have type info[1] you can cross reference the type info with 'generic array' and then do the same with element 2, address 0x34199920 and know it's a 'foo object'.

[1] Say the first 4 bytes is _always_ a type index that the compiler or interpreter generates.