What is an obstacle identification surface and why would a pilot intentionally penetrate it during an ILS approach?
Obstacle Identification Surfaces apply to departures and the portion of an non-precision IAP beyond the Visual Descent Point. If you are in the clouds, you should never penetrate the OIS on departure.
On arrival, the VDP assures obstacle clearance with a normal approach to landing. ILS only approaches do not have a VDP depicted so there is no OIS to penetrate. (The glide slope takes you right to the touchdown zone.) ILS/LOC (and Localizer only) approaches do have a VDP for the non-precision version of the approach. You could penetrate the OIC on a localizer approach and if that happens, visibility minimums are increased as explained in the last paragraph from the AIM below.
AC 150-5300-13-c12 Airport Design has a good visualization of the departure OIC.
(As an aside, it has lots of charts and graphs showing how departures and approaches are designed.)

AIM 5−2−8. Instrument Departure Procedures (DP) − Obstacle Departure Procedures (ODP) and Standard Instrument Departures (SID)

…if an obstacle penetrates what is called the 40:1 obstacle
identification surface, then the procedure designer chooses whether
to:

Establish a steeper than normal climb gradient; or

Establish a steeper than normal climb gradient with an alternative that increases takeoff minima to allow the pilot to visually remain
clear of the obstacle( s); or

Design and publish a specific departure route; or

A combination or all of the above.

The 40:1 obstacle identification surface (OIS) begins at the departure end of runway (DER) and slopes upward at 152 FPNM until
reaching the minimum IFR altitude or entering the en route structure.
This assessment area is limited to 25 NM from the airport in
nonmountainous areas and 46 NM in designated mountainous areas. Beyond
this distance, the pilot is responsible for obstacle clearance if not
operating on a published route, if below (having not reached) the MEA
or MOCA of a published route, or an ATC assigned altitude. See FIG
5−2−1. (Ref 14 CFR 91.177 for further information on en route
altitudes.)



5−4−5. Instrument Approach Procedure (IAP) Charts

h. The Visual Descent Point (VDP), identified by the symbol (V), is a
defined point on the final approach course of a nonprecision
straight−in approach procedure from which a stabilized visual descent
from the MDA to the runway touchdown point may be commenced. The pilot
should not descend below the MDA prior to reaching the VDP.
i. A visual segment obstruction evaluation is accomplished during
procedure design on all IAPs. Obstacles (both lighted and unlighted)
are allowed to penetrate the visual segment obstacle identification
surfaces. Identified obstacle penetrations may cause restrictions to
instrument approach operations which may include an increased approach
visibility requirement, not publishing a VDP, and/or prohibiting night
instrument operations to the runway. There is no implicit obstacle
protection from the MDA/DA to the touchdown point. Accordingly, it is
the responsibility of the pilot to visually acquire and avoid
obstacles below the MDA/DA during transition to landing.

Edit: This AIM is consistent with what the Instrument Procedures Handbook says about non-precision approaches but not consistent with what the handbook says about the potential for obstacles on ILS approaches.
p 7-9

...if an ILS is configured with approach lights or a nonprecision
approach is configured with either MALSR, SSALR,or ALSF-1 lighting
configurations and the procedure has a published visibility of 3/4 SM
or greater, a penetration of the final approach OIS may exist. Also,
pilots will be unable to determine whether there are penetrations of
the final approach OIS if a nonprecision procedure does not have
approach lights, or is configured with ODALS,MALS, or SSALS/SALS
lighting since the minimum published visibility will be 3/4SM or
greater.
As a rule of thumb, approaches with published
visibilities of 3/4SM or more should be regarded as having final
approach OIS penetrations and care must be taken to avoid any
obstacles in the visual segment. Approaches with published
visibilities of 1/2 SM or less are free of OIS penetrations...