We compare because we're the only yardstick we have, and knowing how other animals stack up against us in IQ is extremely important. It not only answers questions about them, but also questions about us. There's nothing purposeless about it. These are vital questions.
If a different kind of intelligence were necessary, that species would possess it - so if it doesn't, it's for a vital reason.
I really don't even know what this means. Humans are pretty darned smart, at least as a species, but we have no evidence that different kinds of intelligence exist. If one does, we need to know about it.
It may seem obvious that each species functions different, but we have no clue, and from all the evidence, it seems likely that the differences are minor, and may even be nonexistent in some species. "Obvious" isn't science, most often turns out not to be as obvious as we thought, and we need to know.
But aren't you curious how animals think, what they feel, how they do some of the things they're born knowing? These abilities are somehow hardwired into the brain, and knowing how this happens could benefit humans immensely.
At any rate, intelligence is what it's all about, and comparison are how science works. We don't just compare an animal species with humans, but with other animals species. If we don't make such comparisons, we'll never learn anything.
The nice thing about being human is that we aren't limited by our own species. We can learn what it's like to be a bird, a monkey, or a dolphin.
