For many type of RF links, if there are long runs of zeroes, or long runs of ones, or if over some number of bits, there are predominantly more zeroes than ones, or conversely, there are predominantly more ones than zeroes, the link error rate increases. Manchester encoding is one means to equalize the ones and zeroes and thus reduce errors caused by lack of DC balance. It also provides a clock recovery means for systems where this may be important (imagine a cheap transmitter using a MCU running from an RC oscillator). However, as you imply, it does not have any inherent error detection or correction properties.