what are the advanatages of using a scrambler
Scramblers are indeed used to "randomize" the data, i.e. to "whiten" the input sequence. The reason why this is necessary is that a transmission system has no control over the data the user is going to transmit. This causes problems because it violates the assumptions that are usually made when designing transmission systems, such as having independent data symbols. Note that usually the data are not random, as you seem to believe. A frequently occurring problem are long strings of zeros in the data, which can cause difficulties in timing recovery and adaptive equalization. These problematic sequences are removed (or, actually, made much less likely) by the scrambler.