I always say that you only hold coins if you have them in your own wallet that you have private key for but if you are lazy or not enough tech savy trusting an exchange during forks/airdrops may be a thing for you.

Currently this fork/split thing is getting out of hand, there is so many coming out it would be a real hassle and pain to get each right away so it might be a good idea to be able to trade fast aka let exchange hold a bit of your BTC.

I remember when BCH forked nobody could deposit or withdrawal coins due to the slow network yet people who had BTC on Bittrex got coins and could trade the markets right away. Risk vs reward as always.

Bitcoin Splits Source 1
[img https://ibb.co/gisYhG /img]

Bitcoin Splits Source 2

https://ibb.co/eF42bb