what is its purpose?do you need it if you dont drive in cold weather?

even during that short time your engine is warming up, you still need the bypass.
it allows block flow when the thermostat is shut.

if you don't run a thermostat, then you could probably block the bypass hose connections.

I've always wondered if that bypass can be partly blocked as it seems like alot of volume moves thru it & a BB has a very small bypass inside the pump housing. yes with no stat you can block it but circle track racing is the only racing (that I am aware of) where you dont run a stat

I don't see what you would gain by not running the bypass.

Bypass is needed. Unless you remove the T-stat.

Cold water in the engine = closed thermostat

Closed thermostat means no flow.
No flow means that the water around the chamber will heat up, but the radiator can't cool it off until the water heat soaks enough to open the stat.

By that time, you've already boiled the water by the chamber and possibly overheated your heads.

Ever watch a pot boil? You get steam bubbles forming at the bottom of the pot before the water truely boils.

Unless you stir the pot to even out the temps so that the water heats evenly. This is what the bypass does. It circulates the water in the engine only until its hot enough to open the t-stat.