When there is just a small breast bud (the nipple sticks out a little) or a small, half-dollar size breast, there generally is not much need for the support of a bra. In fact some girls find that when they first start developing, the breasts are a little tender and sensitive, and the pressure of a bra on top of that can be uncomfortable. The concept of a "training bra" is useful, though the breasts certainly don't need to be trained to stay in place! Once the breasts get to about an A cup size, most girls want to wear a bra, at least some of the time. Some girls prefer to start with a sports bra - it can give some support and allow girls to feel a little protected as they continue their athletic activities.
Some girls want to wear a bra because the bra can "flatten" out the breasts a little and not have them quite as noticeable. I would rather that girls get used to their developing bodies than feel that they have to hide it, but for the very early developer, this may be useful.
I would recommend that you ask your girls how they feel about starting to use a bra. Unless your eight year old is very developed, I would recommend waiting, and just using camisoles. Your 11 year old will probably want to get her first bra, as many of her peers have probably started to use one. This age (middle school) is when many schools start to have more serious gym classes for kids, in which they have to change clothes in a locker room. The experience of changing with a group of other girls can make some girls more self conscious about their developing bodies, and they may feel more comfortable with a bra.