Differences and conversion between encoding techniques (NRZI, Manchester, MLT3)
Multi-level codings allow you to send more bits in a given frequency, with the trade-off of needing more complexity to generate and detect the multiple levels.

At the time the 100mbps Ethernet copper standard was being developed, this obviously felt like the right compromise of interface complexity vs demands for cabling of a particular quality.

Standards like Ethernet are an immensely complex balancing act of backwards compatibility, politics, vested interests and attempts to predict future demand.  You can't really pick one of umpteen overlapping engineering decisions and say 'why did they do that?', because the answer is always at least in part 'because they did all the other stuff'.

In answer to your specific question about recoding stuff, then yes, you can always do that, because if you can decode one format and generate another, then you can put those circuits back-to-back.