I agree with G420Uk above, it's all about controlling the environment. You are basically playing god to the plants and deciding when the sun shines, wind blows, rain falls,.. etc. A tent is a convenient and (sometimes!lol) portable way to create a light-proof, enclosed area that you can then use to house your environment. Before tents, growers used to make their own grow spaces,. there used to be loads of threads on here about desiging a diy grow space, when this place first opened. Another benefit of a tent is that it's reflective on the inside, so it uses your light more efficiently. Before tents, growers used Mylar, or painted the walls of their space white,. Basically, a tent is a ready designed, convenient solution to creating an area to house your grow. But it's not necessary :)
I hope that helped, lol,.. :)