If you make the wrong type of trade you can actually make a huge loss by paying tax. Consider this scenario:
Jonny buys 1 bitcoin in 2016 for R10,000 and holds for a few years. The price of bitcoin increases 10x and now his current bitcoin is now worth R1 million, he hasn't sold yet so does not owe any tax, so far he is only R10,000 out of pocket from his initial investment. He then decides ethereum is going to be the next big thing since 2.0 is coming out so he swaps all of his bitcoin for ethereum.. therefore the current value of his ethereum holdings is worth R1 million. His financial advisor warns that due to the tax rule, he made a taxable income of R990 000 because he sold his bitcoin for another crypto. So he must pay 18% capital gains tax on the R990, 000 gain which equals R178,200. Not wanting to go to prison or wanting to sell his ethereum, he pay this in cash for his tax year. He holds the ethereum for several years but unfortunately due to scalability issues ethereum is replaced with a rival, so the value of his holding drops 99% and now equals R10,000. He decides to sell back to rand to get his initial investment back.
So what did Jonny do, he invested R10,000 in one speculative asset it rose in value, then swapped that asset for another speculative asset and it dropped in value, and then he ended up paying R178,200 in tax eventhough he never saw the million rand touch his bank account.