I believe you are just talking about adding observations to Traditional Projects rather than Collection Project.
One of the benefits of joining either type of project is that the icon of the project will show up on your observation page which may end up revealing a project resource that other users may benefit from by finding out more about a group of organisms or a subject of interest that relates those organisms such as bird window crashes or bird feathers.
As you may well know, Collection Projects add your suitable observations regardless of whether or not you join but as I mentioned, joining shows the icon on your observation.
Qualifiers for Collection Projects may be aggregated by taxa and/or dates and/or place and/or user and/or excluded by the same.
With regard to adding observations to a Traditional Project, you need to be a member to have your observations added. One must also "manually’ add observations to the project - in some cases depending on the interest of the project only certain observations are suitable.
Traditional Project observations may be brought together by special qualities - observed from a car window, observed while on a walk from home, observed on a certain type of plant, observed pollinating, observed deceased (window kill), observed as a blob on the beach, observed banded or tagged, and etc. These qualities are often subjective and need user curation prior to being added to a project.
I made a tutorial that maybe helpful in your quest. Let me know if there is anything lacking or a question that is not answered and I will try to incorporate that. Adding Observations to a Traditional Project - Wiki
This tutorial covers: