Local knowledge. You don't need to search the whole internet for the best things, best nature places to go in Helsinki. You don't need to worry about transport, like how you get to the right train and change to the right bus, a guide picks you up from your hotel and takes you to the best places in nature. You don't need to look for a shop in a strange country to pick up your snacks and lunch for your hike, the guide takes care of those. You don't need to have all the outdoor clothing for every weather, you can borrow them from the guide. You don't need to know where to go, what to pick and what you can eat, the guide knows.