Bicycle shorts are a must for any serious bike rider. You may get away with any old shorts or jeans for a while but it won't take long untill they will bind and chafe and become just plain uncomfortable. On a long ride street shorts are almost guaranteed to give you some chafing where you definitely don't want it.
They don't have seams in areas where they could cause chafing like street shorts do. But the biggest advantage is the built in padding in the crotch and seat which gives you a much more comfortable ride.
Most bike shorts have chamois padding in the croch and butt area but more and more riders are opting for shorts with a gel pouch for padding. Either one will serve you well and is significantly better than street shorts or jeans with no padding and a seam in the crotch area.
Also, bike shorts are designed to stretch and move with you. They actually compress the muscles that you use while riding. Also they are made to be higher so that your back is covered in the bent over riding position. Elastic grippers on the legs to hold the shorts in position.
Please note: Bike shorts are designed to be worn without underwear which caused friction and heat.
If you don't like the appearance of traditional cycling shorts you can still get the same features with a inner liner and outer baggy short. They will still give you the same comfort and support but they will be a bit warmer. Baggy shorts are more likely to have pockets than more typical shorts.
Whichever style you choose your butt will thank you for wearing them.