Why profile code?

What's the point of profilers?  If I have written the code, I know which points are slow, and which do not matter.

>>4
i = 0;
for (i = 0; i < 100; ++i) {
    j = i + 10;
}
print j;

Using this as an example (well, with some higher numbers, and assuming print takes 0 time), I can see that the loop part is the slowest part of the code.  I furthermore know that loops are bad, so I devise a way to calculate j without looping.  Perhaps I can even replace the loop by a constant value.