Plastic roof insulation

[Quintrix]

Most older houses don't have the plastic insulation layer below the tiles intact. Most have seen too many battles over the seasons and are hanging in tatters. I know it is not critical but how important is that layer - blocking dust and water
What are the options and costs of replacing that layer?
Anybody been down this road?

 

[Lupus]

I had it replaced last year, no idea on how it keeps water and dust out, but over 30 years of dust has built up a lot in the roof.

 

[Quintrix]

I had it replaced last year, no idea on how it keeps water and dust out, but over 30 years of dust has built up a lot in the roof.

What size roof and what did it cost you about?
Did you have to do it for some specific reason or just because?