Presale rounds set the cost basis of the people who will be selling to you later. A seed round at $0.02, a private round at $0.08 and a public launch at $0.40 means early backers are up 20x before you buy a single token.
Discount alone is not the problem; lack of lock-up is. If presale tokens unlock at listing, the launch is an exit. If they vest over years behind a vesting-cliff, incentives are better aligned.
Also treat unaudited presale contracts as high risk of outright theft: sending funds to a contract with no deliverable and no recourse is a common fraud pattern, and it is legally unprotected in most jurisdictions.
Related: token-generation-event, vesting-cliff, tokenomics, exit-liquidity