'Drips' in lead box valley gutter-why?

[indus]

Hi

All the details I've read for a lead valley box gutter on the internet state that there needs to be a 'drip' after each 1500mm for code 4 lead.

I'm trying to understand what the 'drip' is for. Is it to allow for thermal expansion or does it help to allow the water to fall?

Many thanks.

 

[indus]

Please ignore, I've found the answer on the web and it is for thermal expansion.