In a nutshell, projects stand alone. Campaigns may stand alone or have other projects and funds tied to the campaign
Using campaigns will allow you to segment your giving opportunities into specific areas of interest. This will increase giving to the campaign while honoring the donor's intent.
For example: you are running a three-year capital campaign. You can display this campaign for online giving. However, if the campaign has three key objectives (sanctuary roof, parking lot, nursery expansion), you can create a project for each of these objectives.
You then add each of the projects to the campaign for reporting purposes, while displaying the projects on the website independently of the campaign. You will have some people donate directly to the campaign while others will donate to the specific project they wish to support. If I volunteer in the nursery, I will probably want to support that project. My statement will show I gave to the nursery project and the church will see that as progress towards the campaign goal for reporting purposes.