Mainnet means a project is ready to use their own network or blockchain, majority of the projects in raise through ICO and it's easy for them to create tokens for sale under the ETH network, but as the development matured they create their own network.
some of the successful projects that runs in their own network now are Binance and TRON, I have invested in these projects prior the mainet and I can say I'm one of the lucky investors who enjoyed a decent profit.
The impact usually is the price will increase due to the hype, but provided it's a popular project.
And to add, there will be increase if they have really good use case in the market. Even if the platform is very noble but getting users is hard, I don't think they will perform good in the market. The use case should be usable by many even non crypto users. In that way, they will create demand for the coin, thus, be valuable in the market.