This is a resource-consuming operation. Enable this property only if you often get e-mails with Lf instead of CrLf and the container where you display the plain-text data does not understand Lf as a line break. For instance, the standard control does not break the line when it encounters Lf without preceeding Cr, while RichTextBox does.
If you know you need this conversion enabled, be sure to enable it BEFORE accessing any properties or methods of the downloaded message (i.e. before the message got parsed).
This property affects only plain-text body. For HTML, the exact format of line breaks is not important.