Image Verification is needed to beat attackers,
who otherwise might try to add thousands of email addresses
to emailing lists around the world, by use of automatic programs.

Their programs can not read the text in the dark gray rectangle,
because the text you see is stored as a picture and is not stored
as characters.

Hence you, as a genuinely interested person, can type in
what you see, and join the list with this as authentication
that you are really interested.