We all know that Crypto is a long term investment but not all.
Only Bitcoin have that guarantee of been hold for long term, other coins too can be hold but the risk is very high.
Many people buy memecoins and some other coins with no use case to make money but they listen to influencers and keep holding after making huge profit to later end up at loss. Always remember that these Altcoins cannot continue to go up forever. Selling at profit will surely make you buy more at low price if you believe in the project.
Also, selling is important in most cases because its same as taking profit which is risk management, brings peace of mind and lock you in gains..
Selling at the right time also allow you to diversify your portfolio.
These cases are mostly for altcoins.
Some projects do have hype from the community, and as a result, some investors will buy the coin and sell it at the right time or when they feel it is the right time to sell, and once the major investors have sold off their coin and taken their profit, the project will go down, and people that are still holding may end up selling it at a loss.
This is the effect of not doing your own research about a project before investing and due to hype, and this is only applicable to centralized projects. That is why, if you want the rest of your mind in cryptocurrency, invest in Bitcoin and rest.