If a person shows his/her photos covered with clothes in the adult
website, what is the 'meaning' of that purpose?

FETISH

If I understand your question correctly then there really is no
definitive answer.  A web site can be adult without being
pornographic, a person may simply not wish children on their site for
whatever personal reasons they may have.  Or, as the other comment
suggests it may well be a fetish type act.  Since people are free to
set up their web sites however they wish you cannot pinpoint an
individual's motives without asking them directly.

I owned an adult fetish website for 3yrs. I now own www.tramplegoddesses.com
Even though these are considered "adult" in nature, I have NO nudity.
As a webmaster I see no point in it. As theoracle said, you can have
an adult site without nudity. I won't lie...there ARE many sites in
the same fetish area as mine who have full nudity. Mine just isn't one
of them by choice - and this has drawn many visitors to the site
because I took that stand.