what exactly is "custom e-mail from headers"


i checked the "usage page"
but did not really see any further explanation.

i see the options are "E-mail Address" & "Sender Name".
WP "network" and "main site" already has settings to enter email addresses that WP generates for automated emails.
so is this to over ride these settings for WP automated emails (sign ups etc) ?
if so what sense would that make ?

checking the forums i saw mention to SMTP but i cannot understand the relation here for that if there are only settings for "E-mail Address" & "Sender Name" as mentioned.

kind regards.