Everyone that has run an ICO or that has been fully involved in running an ICO knows that it is a battle between your product development and marketing your ICO. Primarily because finances are limited and of course time is always moving.
However what I have observed is that there is a possibility where each complements the other. I believe that if you have a great project, and can bootstrap or find a team that believes in the project, you can cut your expenses on human resource and push the payment forward. This will leave some funds to push the project and the ICO which gives prospective buyers a boost of confidence.
The ICO space has seen some really great ideas taken off and running well and some even more number of not yet successful or dead projects. This has reduced buyer confidence and has (I believe) helped a good deal to separate the wheat from the chaff.
Having a product working before the ICO is a big plus. It separates you from the rest of the ideas - ideas - ideas group by a fine margin. Your marketing now has a lot less to do in convincing people than without the MVP.