I live in the Southeast, and I tarp camp.
Basically, the lines between these things are blurring, and some tarps are becoming "shaped tarps" which are getting closer to tents, and tents are getting lighter and more open and closer to tarps.
A lot of them are meeting in the middle at a form of inner net-tent and outer rain fly. Depending on how far the side walls extend to give all-side coverage might make the difference between calling it a "tent" or a "tarp".
So, alot of people just use the general term "shelter'.
You can get the basic necessities for camping from many different types of these shelters, and as you noticed, needs can be different in various areas of the country, and in various seasons.
The reasons why people want inner net tents and outer rain coverings that there is a pretty common need for bug protection and rain protection. But not necessarily all the time. For example, even here in the southeast, bugs are not a problem all year long. There are bug seasons, but just last week I spent the night out without any bug net because it was cool enough that the bugs weren't out. This allows you to leave the bug net home, and just take the rain fly, which essentially is a "tarp". In other areas of the country, there isn't much rain. But sometimes people want to have bug protection or from creepy-crawly critters getting on them, so they bring the inner bug-tent only, and camp "under the stars" but with their netting protection around them for their comfort. Others sometimes just sleep in their sleeping bag "cowboy camping", or they just use their bivy alone. Some people like to feel unrestricted and closer to nature, so they use the minimum.
In most all situations on this website, people try to minimize the weight and bulk of what they are carrying, so they can hike lighter and go further and faster. Many are long distance hikers who are hiking the Appalachian Trail or other long trails.
In the end, we have a saying, "Hike your own hike". You do as you wish. We discuss a lot of things here so that we can all share ideas to make our own hikes as pleasant for us as each of us desires, in our own ways.
You will find hikers here from all areas of the US, and even around the globe. There is a lot of info to share.