There are a bunch of different reasons you might want to go ahead and jailbreak iOS 8 on your smartphone or tablet. The two biggest reasons I can think of is to get rid of bloatware on phones, and to break the locked down control that the manufacturer has over how you can do things such as transferring files and music. You can only do things through their proprietary means if you have a locked phone. If you have a branded phone, you know it is loaded with a bunch of junk software that you cannot get rid of.
Now as for tablets, if it is Wi-Fi only, then the bloatware is not a problem. Just manufacturer programs you can pretty much easily manipulate. However, if your tablet is also branded by a cellular provider, you have the same problem with the added bloatware that phones get. Of course though it does seem that the makers of the iOS products have been kind in not allowing cellular providers to get too carried away in adding the junk software.
Another reason to jailbreak iOS 8 is if you want to use it on any carrier. If you have a phone that is already tied to a carrier it is pretty much as easy as swapping out a SIM card to get it to work on your account. However, we do know that there are phones out there you can buy that are not tied to carriers. You buy them directly. They do not have junk and they operate fine as soon as you plug in your SIM card. To get a carrier branded device to have some of those freedoms, you would need to jailbreak iOS 8.
If it is your first time doing this, you probably want to try it on a phone you could afford to lose. You probably have one in a drawer somewhere, or a relative has one. Maybe you have one with a cracked screen that still works. That is the type of phone you experiment jailbreaking with, not your brand new one.