Subnormal numbers have a different, basically fixed-point, representation. They exist in order to bridge the large (relatively speaking; indeed "infinite" in a sense) gap between the least positive normal number, zero, and the greatest negative normal number, caused by the usual significand-exponent representation.

Most "mundane" uses of floating point have no need for subnormal numbers, and numbers that underflow could just be flushed to zero. But they’re important in scientific computing to ensure sufficient smoothness around zero, avoiding precision issues.

I don’t know if any bugs contribute to this but this in the intel case but it has been very common historically for subnormal performance to be lower on many processors, and things like the Alpha required you to handle them in software if the COU fired a trap.

Have a look at https://en.wikipedia.org/wiki/Subnormal_number for some context.