Title is a badly worded question, on which I do not have enough knowledge on the subject to answer on my own. Anyone to help out?
Scrambler is a randomizer with a purpose to encode transmitted data so that there will be no long sequences of only zeros or only ones in transmitted data. It is done to support proper timing recovery, when receiver waits for bit transitions to synchronize. Scrambling is usually done for channels with self-synchronized line codes, e.g. ISDN u-interface and Ethernet.
Generally, scrambler doesn't need to change the amount of transmitted bits, hence it doesn't change the speed of data transmission.
You can compare a scrambler to error-detection and correction schemes, which add redundant bits to support its functions. You can similarly compare it to 8b/10b encoding, when 2 bits are added to every 8 bits of data. As a scrambler doesn't add additional information to the data, it is not a redundant process in that sense.
Also, probably irrelevant in most cases, but it can be considered redundant, when used for a signal that inherently doesn't need scrambling. For example, UART uses short transactions and receiver is capable to get all bits only based on knowledge of a baud rate, so use of a scrambler would be redundant.