And you cannot do that with a map and compass?

Nothing wrong with doing that way. But even when you can (and you can't
always, consider VFR on top), the GPS is more accurate. And when you
are flying over Canada, over unfamiliar terrain, it gives a level of
confidence that you don't get by map and compass. Compasses are subject
to magnetic anomolies, and many, many, aren't that accurate. When
coming into Las Vegas airspace (and never been there before) and I see
that the airport is 7.3 miles dead ahead, but still can't see the
airport, I KNOW where I am, where to fly next with ease and precision
you don't get with a map and compass. And I don't have to divert my
attention from flying the plane, just a glance at the GPS and I KNOW.

Also, it makes flight planning a breeze. No legs to figure. Just click
in your route, check for restricted/prohibited airspace and TFR's and
go direct! I have a Floatplane and I have put the Floatplane landing
lakes (and there are lots without anchors that you see on the charts),
into my GPS, so I have those without consulting some archane book.
Also, like I said, I have the names of the rivers and highways at my
fingertips for everyone to enjoy.

I flew direct (good to save fuel) from Sioux St Marie to a small
airport 200 miles north of Montreal over terrain that was remarkably
difficult to get good visual fixes on, and out of range of VOR's. There
were lakes, but one lake tended to look the same as the next. Other
than that, just rolling terrain that was endlessly similar.
Occasionally you would see a road or railroad (like every 150 miles),
but lets face it . What with tight on fuel, and such, I don't think I
would have gone direct here without a GPS. Too risky. I would have
diverted to the airways (which were NOT convenient), used more fuel and
taken longer.In that case enough longer I don't think I would have made
it in one day. That would have meant finding and airport with a hotel
and all that.

There seems to be some macho, hair on the chest attitude about not
using the GPS. Lots of instructors that ignore it's existence, won't
let their students use it, but lets face it, there isn't a pilot out
there that actually goes anywhere cross country that doesn't have one
and use it. No reason not too. They work great! There is a need to be
able to navigate by other means as there is always a chance the GPS
signal will not be there or your unit will malfunction. So cross
checking with other navigation techniques periodically is still a good
idea. I have over 2000 hours behind mine though, and have yet to really
loose a signal (a couple of times, temporarily, and I suspect it was my
unit, not the satellites).

When I land in a strange town and borrow the courtesy car (or rent
one), and want to see the sights, my Garmin car unit tells me where the
museums are, where the resturants are, where the rivers are. Great
little device for that too.