Well, using a tarp is somewhat of a personal choice. There isn't a lot of difference between a tent and a tarp. Basicaly, they both provide a roof over your head in bad weather.
Besides the roof, and staying dry (which means good ground selection as others have said) there is the floor and body. Dropping these two means a lot less weight, but it also means good ground selection skills, and, knowing how to deal with bugs, and critters.
Ground selection is a bit of a chore. Sometimes you don't have a lot of choices. For example, I drive up to the ADK's ready for a two week outing. But it takes about 5 hours. My wife comes with me. So0, I am often stuck with a site that I don't really consider good. The wife doesn't care to sleep under a tarp (though, I don't understand why, there really isn't any difference between sleeping under a tarp and in a lean-to.) So we have two shelters. One is the tent for car camping and one is the tarp I will take on the trip. No choice with site selection means IFF it rains I need a good floor to stay dry. Out hiking, I can select a water resistant site, not one with pools or heavily saturated ground, but one with a small mound to sleep on. I use my pad as a ground sheet, even if the ground is wet, I stay fairly dry. I have managed with a down bag and down jacket, through several days of rain. It works, OK.
Bugs and critters can be a real problem. Near Little Falls, NY, there are quite a few Timber Rattlers...near Lake George, some of the southern tier, too. They have never come under my tarp. I don't think they really care about water, they care more about temperature. Tarps really are not that warm compared with a closed in tent. So, they are not atracted to the tarp. Bears are not really a problem. A tarp is just large enough, and smelly enough, they do not usually bother with it. But, it is safer than sleeping in a tent if a bear DOES visit... 'Coons can be a real pest. Bold and always into your stuff. Mosquitoes are perhaps the bigest downside to tarps. If temps are much above 60F at night, you need to use some sort of netting. DEET only slows them down, it does NOT prevent them from biting. Nor does permethrin, often they bite first and die later. Pesty basserds... Spiders, centipeds, and other bugs can also be out there to bite if you lay near them or their lairs. Avoid sand fleas/bed bugs near water sources. I got some in my bag that prompted me to spray it when I got back. An insect pad or two can be carried to eliminate them, if you stumble across them.
Anyway, this is part of tarp camping. Only in summer does the night time temps stay high enough to make netting required. I prefer the tapered A frame or diamond as a shelter against rain/wind. In nice weather, you can set up as a lean-to, sleeping towards the back. Check the sky first, of course. My total weight, including stakes, guy-lines is about 14oz. A 5'x4' piece of netting adds another 2-3oz, if needed. Again, ground selection is a critical skill to master. A mound, a slope, and under a tree (using it as a set up point) is my first choice. Second is a more open campsite with good drainage away from where I want to sleep. I would rather have a few roots under me than sleep in a dip. IFF I have to, I will pile about 6" of forest duff under me (not exactly LNT) to sleep above the local water level. You really only need about 20"x72" to sleep in. Sometimes between some bushes, works OK, and makes good tie-offs when I'm canoe camping.