Does it bring more security, does it take away freedoms, is it to stop terrorism or something else? And lastly, is it worth it, and is it successful in what it does?
In order, No, Yes, It's to show people that Things Are Done, No, and No.
To elaborate, does it bring more security? No. Because Terrorists, as idiotic as their basic mindset is, aren't dumb enough to plot schemes that can be foiled by existing security measures. There just is no security procedure in the world that has no holes, or that cannot be subverted in some way by a person with enough determination.
Does it take away freedoms? Yes. By creating an air of insecurity and apprehension around the idea of travellling to other places, our freedom to make decisions is taken away on an emotional level.
Is it to stop terrorism or something else? Well, kinda. Personally, I am more convinced that this is more a case of politicians feeling the need to show the screaming public (or, more accurately, the media) that Things Are Done to make stuff more secure, instead of going with the established methods and procedures.
Is it worth it? No. See above; The only effect this whole security theater has is to weed out the kind of amateur idiots that would have been caught by less elaborate measures, or better trained personnel anyway.
Is it successful in what it does? Well, they certainly claim it is, based on the evidence of there not being any repeat "commercial airliner crashes into national symbol" incidents. But again I come back to my point about the people who plan major terrorist acts not being that stupid, trying the exact same trick twice isn't something you'd find in a terrorist plan. However, going by the number of incidents actually prevented by the TSA (little to none) vs the number of complaints regarding false positives and general mistreatment of regular people (lots and lots), I'd say their overall efficiency is rather low.