a) They can access a broad market of potential investors who, simply for the sake of promoting such ICO, need to be informed about the advantages of investing in that project, which is the first step in convincing them to invest.
b) They promote in an extensive way the brand announced among the thousands of users of the forum who, otherwise, would not have knowledge of that brand.
c) Thousands of dollars are saved in advertising and publicity since developers usually only dedicate a minimum percentage of their tokens to this type of campaign, often less than 1%, when a massive campaign can often cost 40 times more.
1) You are right about this, the signature campaigns gives them early exposure and helps to gain initial users. They can show this while applying to a potential investor.
2) It is obvious that any project being advertised here will attract a lot of eyeballs. Main thing is how many of people convert.
3) This being a tried and tested method, developers are now not scared to spend on this. They are giving away some tokens for free but it is still an added cost to them. Those coins could be worth thousands of dollars later when the over all value rises.