The main aim of ICO is to raise funds for what ever idea they have to deliver the product. This is the main motto of every ICO. So they maintain different strategies like Private sale ,Pre sale ,Main sale
To address this question the pre sale is the stage where every ico keep max off to purchase their tokens and eventually decrease the offer as the days proceed and here they implement smart contract technique to send the tokens to the investors who have invested in their ico. But many ICOS implement the smart contract technique at the stage of main sale.
So during the main sale the value of token of the particular ICO increases and the demand also increases too. And the tokens will be distributed to the investors to their wallets after they have purchased.
I personally feel that Presale is the best sarele where every one can get huge number of tokens for just investing the small amount.
I will take the example of one ICO. You will get entire knowledge of the Presale and the Mainsale
SwachhCoin which is a Decentralized Waste management sector is now is now in presale phase where they are providing a very huge discount on the tokens.
They are providing around 47% discount for the 5 days of presale then the discount automatically decreases after few days. Which is very huge for the crypto investors and eventually they will come to stage where there will be not discount for purchasing tokens.
In this phase they accept currencies like
Btc,ETH,XRP…etc and some accept fiat too.
Good luck!
Note :- I am not an investment advisor. Do your own research before investing.