Depending on your scale, typical uses are brake lines, small fuel lines like Nitrous, oil lines, gauge and other control lines, etc.
The scale size is determined by its outside diameter (OD) compared to real world braided line since an inside diameter isn't useful in scale modeling. See the tables in the section called LINKS, INFO & TECH.
This .018 diameter braided line equals a scale equivalent size of:
1/25 scale - 3/16" or -3 AN