As for why companies/universities do that? To prevent theft.
I know why they do it, and I understand the rationale behind it. That Dufferin-Peel decal that is stamped at the back of my Amiga 2000 has a 5-digit code that is used to identify the unit, and can be punched in by an administrator on any school computer to find out where it's located, what service has been done on it, etc. Now that the unit belongs to me (it was given to be by an ICT admin whom I've known since I started high school), the code is useless, and has been wiped from the board's inventory list.
My argument is that an expensive piece of equipment does not have to be over abused with unnecessary paintings, sketches, etc (although mcdermd offers a great exception to that statement). A simple identification code - such as the one I've shown - is more than sufficient. The only advantage with ruining the equipment's cosmetic appearance is thieves may be less inclined to attempt to take that unit, then one with an identification tag.
They don't even think about resale value.
Some schools may decide to sell off their old A/V inventory (with the permission of the board admin, of course) to the general public to raise funds, etc., as some of the equipment that has been forgotten/retired is hard to find (my school still has a 16mm film projector in their A/V room, a sound board from the early 70s, and some rare Amiga-branded monitors, amongst other things), and thus valuable to collectors, etc. It's better to resell than to throw things that still work out, which, unfortunately, is the case most of the time. That's were people like you, and me come in (the "liberators") :beige:
Perma-stick metal tags or melted-in stamps are effectively permanent.
The identification tag I've shown will not come off, and I have no plans of doing so, as it will ruin the paint finish on the majority of the equipment I've received over the course of my secondary school years. These tags don't bother me, and they have no effect on my equipment. In an odd way, they're almost like keepsakes.
This is what it looks like
Nice iMac, btw, quantumii. Hopefully the plastics are not becoming overly brittle.