whats the start button for ???

I'm going to take your question at face value, although I suspect there may be some deep sarcasm that escapes me. Micro buttons are quite common on high end motherboards for Power, Reset and Clear CMOS. They are dead handy if you do a dry build of your system before you plug the components into your PC case as they allow you to tinker with settings and configure the BIOS without connecting the case buttons to the front panel headers. Micro buttons also avoid the need to short the headers with the blade of a screwdriver to turn the system on.

Why on earth would you need to do a "dry build" outside of the case? What settings are you playing with that requires it to be outside the case? I generally configure the BIOS with a keyboard myself. On high end mobos the bios reset switch is usually conveniently located on the back plate for any screw-ups.

Have always managed just fine with a screwdriver to jumpstart it - or indeed, an actual jumper. I often find I have a screwdriver to hand when building a rig :-)

I could forgive the button, were it not big, red with "START!" emblazoned on it.

Annihilator (Annihilator? Really?)

If you're building a new PC it's much easier to plug the CPU, CPU cooler, memory and graphics card into your motherboard and then install the OS and drivers with the system flat on the bench. If you suffer a problem such as incompatible memory or something silly like a duff cable it is much easier to sort it out on the bench than it is inside the case. If you get into overclocking you'll find that some motherboards will reset the BIOS after a problem while others will lock solid and necessitate a reset. In the latter case micro buttons are a Godsend.

I want a motherboard with a big red start button on it that I can't see in normal use.

Just knowing it's there will increase the stability of my experience.