I completely agree with Jibaku, the megas are there for support:Why does the Big 6 team have two megas on it? Is the idea to pick the mega depending on what you see in Team Preview?
Kangaskhan provides fake out support, which is great set up support for talonflame/xerneas/smeargle, as well as helping out groudon in mirror matches etc
Salamence provides a check to mega rayquaza and primal groudon, which rounds out the team nicely. It also provides intimidate support, which is great for a set up xerneas with +2 SpD. It also provides offensive pressure with a spammable move in hyper voice, which is really useful alongside xerneas etc for chip damage