I agree to the above 2 posts; but I will still answer your initial question. Obviously, the main use for a CB is for communications. Depending on your antenna, you can ussually reach out 1 or 2 miles without issues. With a good antenna, you can extend the range quite a bit, and, when atmospheric conditions are good, you can go for hundreds of miles, but you cannot rely on that as it does not happen all the time.
For regular communications, you can use the regular CB, or Single Side Band (or SSB), which allows for more reach than the standard CB communications. Most clubs and CB users (4X4 users, I mean) do not use SSB, but some other do, and it can be useful to have.
Another thing some CB's can do is receive weather information (WX channels) for your area. Again, useful for when you are out in the trail.
Your CB will be limited to 4W (FCC restriction), so you will depend on your antenna to determine your reach. This means that a well tuned antenna will get you a much larger range than an antenna that has not been tuned (technically, you could damage a CB if you do not tune the antenna, as the transmission is reflected to the CB). Of course, the best antennas are also the longer ones. I run a fiberglass one, and it still extends about a foot higher than the roof of my hardtop. Metal whips can be twice as long (but not necessarily). There are many options, and you need to research whats right for you.
On the not so legal side of things, you can also run a "brick" (linear amplifier), which would extend your range for transmissions, but unless the other person has a linear amplifier as well, your reception range will remain the same, which makes the point kind of moot. In my case, I DO have a linear amplifier, but rarely use it, since I also have a HAM (2M) radio which I use way more often (2M technically is shorter range, but with the local repeaters, it is actually longer).
Hope that answers your point....