Product review (5 stars): A wonderful idea.
This allows you to switch from venting your dryer outside in the summer to venting it inside for the heating season - with a fine screen lint trap for the inside outlet.

If you vent outside, you're throwing away moist warm air.

This is a dirt cheap way to save a little on your heating bill and get extra humidity in the air.

They're very easy to install.  And once it's in it'll last for years.

You'll probably save enough to pay for it at least once a year.