If the project think that they need some fund to continue their project then IEO is the best solution. Many project have their own fund to continue their project so in that case they don't go through IEO they directly try to listing an exchange. Many people trade a single good quality exchange so if a project try to start their IEO on exchange probably many people know about that project and will try to invest on it. You can all it marketing strategy though cause many projects doing this although they have much fund to run their project. You can instantly have an exchange for your token also.

Other side simple listing token are working much silence than IEO projects. Those exchange are much better than IEO projects cause they can handle bad situation better than IEO projects. And in my experience this kinda project have better future that IEO projects.