I use it to clean apk and other downloaded files. Other than that will the android take care off. And no, it can't hurt any performance. But it's not necessary at all to use it.
I had clean sweep (or something) on my note 3 it was fine, when I put it on my note 4 it deleted apps all the time. I had to get rid of it, not worth it.
Galaxy Note 4
Galaxy Tab Pro 8.4
Galaxy Tab 2
Galaxy Gear S/ Gear 1 Tizen
Galaxy Circle
I use clean master but honestly my phone hasn't lagged much even when I have multiple apps open. I was dreading the infamous touchwiz lag upgrading from my S2 to Note 4 (the S2 was horrible for the lag BTW), the phone has only lagged on me a couple of times in the two months I have had it and it's been bad apps both times.