A very good question, indeed!  One, I think we as research administrators
should keep asking in order to understand our role.

In keeping with the [philosophical] & simplicity of your question, my two
"simple" rationales are as follows (in the order of priority):

1. To help.  Theoretically we can spot things which might make proposals
less competitive (like a totally ridiculous budget including many items
which are not allowed by the funding source, thereby implying that the
person requesting the funds does not understand the funding source goals of
the program).

2. For the university's protection.
A proposal is an implicit agreement and committment of funds to perform x,
y & z services.  (an example) If an award is made & we suddenly discover that
the P.I. has included all sorts of matching funds which are not there, it
puts the whole institution in a very precarious position (perhaps legally)
as well as screwing things up for future applicants to that sponsor.

******************************************
* Jane E. Prudhomme, Director    *
* Research and Sponsored Programs  *
* University of New Orleans        *
* New Orleans, LA 70148            *
* (504) 286-7154                   *
* Internet: xxxxxx@uno.edu                *
*  Web Page: http://www.uno.edu/~orsp    *
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