Proposition 3.1.6:
A is closed iff A contains its limit points

and the def of a limit point.
<https://en.wikipedia.org/wiki/Limit_point>

I usually think closed sets have limit points
because of 3.1.6.
So l recently thought of open sets, and
found they do too.

If both open and closed sets have limit
points why is 3.1.6 useful?

Where is the defining difference in using
it

Ok . I thought more. An open set could have more outside the
set