Holding makes you owner of your funds and that ownership is what makes it easier to understand what bitcoin is actually. If I’m going to trade my bitcoin or may be invest it somewhere then it’s going to be just another fiat ecosystem. Having a wallet with private keys makes me owner of those bitcoin. I am not loosing any value if I consider one bitcoin equals to one bitcoin. I only lose it if I sell it or send it to someone or maybe consider it’s value in terms of fiat. Anyways that’s another story but Holding is MUST at least with my own experience.