Ok, first the basics
If you turn on the tap, just a dribble
And hold a straw, just off vertical underneath
The water will flow to the end of the straw
And drip off

Imagine many straws, densely packed
Just the tips showing
All sloping at an angle
And fixed to a steep roof

Water (rain) will be shed
And the roof will remain dry

The steeper the roof
The quicker the rain will shed
But the steeper the roof
The more material is used

Then there's the thickness
The thicker the better surely?
Well, the thicker the layer of straws
The flatter the angle at which they lay
And so the less efficient they are at shedding water

Thatching
Like life
Is a compromise