When travelling with a partner, you are never alone and you might feel safer. You can also split up,e.g. if one wants to go to the supermarket, the other one takes care of the luggage. A great advantage definitely is to save weight as you can share certain equipment such as a camping stove or tent. The units you buy in supermarkets are often too much for one person, so share it and save money.
But you should always talk about your itinerary before you start travelling, otherwise you might get into hard discussions.
In a group, your trip can be a lot of fun OR become very exhausting. Depending on how well you get along with eath other. An advantage of a group is that you can split up anytime. When it comes to activities you want to do in a city or concerning the route you want to travel.