sitePublishing.idx.php - can the cache value be changed from default of '0' ?

This file appears to have something to do with cache management and the default value is 0

What will happen if I changed this to 100 (for example)

Will all cache flush every 100 minutes or 100 seconds or will the sky fall on my head?

No amount of Googling yields any clues as to what this file does exactly.

Anyone know?

Yes, this file contains cache management data, i.e. parts of your pages.
You should allow MODx writing to it, by setting permissions (chmod).