Why does a stored proc become invalid

[chikkodi]

When I looked into the utilrp.sql script, it does the following

1. invalidate all pl/sql modules and recompile standard and dbms_standard
2. recompile all invalid pl/sql modules

and they say you have to connect as internal to do it.

Does this script invalidate and recompile "ALL" the pl/sql modules in the database ? If so, isnt it an overhead ?