The S8 doesn't have an option to remove the battery. I personally prefer having an option to replace the battery rather than needing to rely on a battery pack when I go on a trip and plan to use the GPS a lot and take many pictures and videos. Of course this is just for specific purposes and many people might wouldn't even take advantage of a replaceable battery if there was an option.
The thing is that it's beneficial or a heavy multimedia device like the Galaxy S8 to have an option to replace its battery. The problem is that when you make a phone sealed against water, you have to give up on the removable battery option. Those who will buy the S8 will no doubt take advantage of its high performance and camera capabilities for taking pictures, recording videos, playing games and browsing the Internet on its stunning display. I know that the SD835 is very power efficient, but I think that a 3000 mAh battery is a low for this type of high-end smartphone device. I'm pretty sure that if there was an option to add more juice to this phone, Samsung would have done it. I think the reason for the relatively low battery capacity is due to the limited amount of space existed when Samsung decided to go with such design and advanced features.
According to the official S8 battery specs, we are talking about up to 20 hours talk time, up to 14 hours Internet use over WiFi (up to 12 over 4G) and up to 16 hours of video playback. These are good numbers, but if you play a lot of games, record videos and use the GPS, you'll notice that the battery will be drained out of power pretty quickly. The other good news is that the phone supports Quick Charge 4, which means that you can charge the phone from 0% to 100% in around an hour and a half more or less.
The Samsung Galaxy S8 has the same battery capacity as the Galaxy S7 (3000mAh) but lower capacity than the Galaxy S8+ (3500mAh) and Galaxy S7 edge (36000mAh). When you look at the numbers, there S8 battery life is slightly better than the S7 for Internet use over WiFi, and for watching videos, but more or less the same, even slightly less, for talk time and Internet use (3G/LTE). This means that if you find the Galaxy S7 battery sufficient for your uses, you probably will be satisfied with the S8 performance. If you don't find it enough, you probably want to look at the S8+, which comes with a more powerful battery and longer battery life.
As I mentioned earlier, it entirely depends on what you do with the device. If you are playing a lot of games and use the GPS on the go, you will feel that lack of power pretty quickly. Even the S8+ can't compete against the iPhone 7 Plus in terms of battery performance. The more tech you have on the device, the more power it will consume. Having an option to remove the battery and replace it with a second one is really helpful. The only other option is to purchase an external battery pack (I have a 10,000 mAh from TSST which I bought on my vacation to Barcelona).
Carrying a battery pack isn't such a convenient option, because you will need to connect the phone to the device and charge it as you use it.This means that if you really need power now, you will need to hold both the battery pack and the phone. You can put the battery pack in your pocket with a long USB capable and use the phone like that, but it's uncomfortable and I'm not sure it's even healthy and safe to put it there. You can just charge the phone using the battery pack while not using it. When I travel, I carry a backpack and just put it there and take it out after some time. As I mentioned, the good news is that it can charge up very fast do to the Quick Charge 4.0 technology, so it's not like other devices that will charge much slower. You can always charge it in a McDonald's while eating your delicious hamburger. By the time you finish, you'll have plenty of juice.
If there was a replaceable battery option, the only thing that you would need to do is to just pop out the empty battery and replace it with a fully-charged one. You could even buy more than one backup batteries. It's much more convenient to have a removable battery option than not having it. This is on disadvantage that the Galaxy S8 has in my opinion, but again, you might not see it as such a big deal.
By the way, there is an option to replace the battery, but you need to disassemble the phone and you probably lose the warranty, the water sealing and who knows what else.