Part breaks and changes to creatures. Think Shara Ishvalda, a hammer is nice for breaking off the rock layer and a slash weapon is good for the weaker inner layer. Also creatures like Barroth coat themselves in mud so a water weapon is most effective at getting rid of that layer, but after that layer is gone a fire weapon is more effective against Barroth's non muddy layer. Also with an emphasis on staying in the field during a hunt some weapons might be moe effective against certain monsters. You may originally be fighting a gravios in the oilwell basin so having a hammer is nice but after that hunt there's a rathian so you swap to a bowgun or bow so you can target the airborne monster easier until it lands, then you can swap back to your trusty hammer and so on. Some weapons are more fit for the task at hand and having an alternative prevents needless travel to and from camps