ICO stands for Initial coin offering, so it's only feasible if you're creating a new coin or payout coins from the dev wallet.
It is possible to raise funds for an ICO as an already existing company, yes.
Indeed. Thats why they are creating them and to be honest thats the easier way to go as compared the bank funding option. The later one needs huge documentation, collateral securities and much more to pass the funding.
ICO's first principle is to create the funding source and thats why they provide you token of share! That symbolises your investment and give you title of share holder, thus once they profit then they share you profits!