Tom Young writes:

Flaps have two main effects, they increase drag, like you said, and they
also increase lift and reduce your stall speed. These two effects can be
useful at different times. For example, if your wing is on fire and you want
to descend quickly, you lower the flaps and descend at Vfe (max speed with
flaps extended, top of the white arc). It's an interesting exercise the
first time you practice it, with the ground filling up your windscreen like
that and all. Another use is when you simply want to fly slower, such as
when you're in the traffic pattern and want to avoid getting too close to
someone in front of you. You can slow down without flaps, but using them
keeps you farther from a stall, which is a good thing at pattern altitude.
I'm sure more experienced pilots can give more interesting ways to use flaps
in normal flight, but those are the ones that come to mind just now.

In simulation it seems that flying slow with flaps extended makes the
aircraft more prone to exaggerated movement for a specific control
input, especially in larger aircraft. Is an aircraft inherently less
stable at slow speeds with flaps extended? If so, is it just because
the flaps are out, or is it the slow speed that does it?

When flying around a city for fun at low altitudes (2000-3000 feet), I
often fly with full flaps and throttles near idle. It makes it easier
to go slow and enjoy the view, but I also get the impression that the
envelope of safe maneuvering is smaller in this configuration. But I
don't know if it's the flaps that do that, or the slow speed, or
perhaps both.

When I first tried simulation (many years ago now), I was surprised by
the drag effect of flaps. I had read about it but I didn't realize it
was so significant. I finally understood why I had heard commercial
pilots increasing engine speed while approaching a runway on landing.
I find myself doing the same thing, to maintain altitude mainly, and
also because it seems to improve flight characteristics if one flies
with more power (?), as opposed to just gliding in at the slowest
possible speed.

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