Guess they didn't want to face the muffalo police.
Honestly, I find it to be something of an undercooked feature, and hope that the world outside one's home tile gets built up a bit in a future update.
You can settle new tiles for: Getting away from a threat that's too big. Infestations are a common cause for this. Cheesing resources - You can quickly grab up all the gold and silver of a map before leaving and settling a new tile. Sell at neutral bases for shock lances, horses, and food. Tomb raiding - Crack open those ancient dangers to very quickly level up your weapons and armour, as well as a consistent source of prisoners and bionics.
So new tiles can trivialise the early game gearing up and recruiting process, and at this stage there's basically no consequence to it, since each tile gives you some grace time to settle. To a lesser extent it trivialises the mid-game rush for steel, because why scan when you can go to one of the 6 adjacent tiles and load up a herd of muffalo?
So all in all I feel like settling new tiles is OP in a broken way, and I'd like to see some more consequences. Ideally, threats should be based on faction wealth, not colony wealth, and start appearing after game start, not tile settled, and some threat types should be able to chase your caravan on the world map.
Oof, yeah, plague is a nasty one to start off with. Generally the very first "mission" I do after setting up survival essentials is kill and butcher a couple of big animals, then try to sell the meat and leather for penoxycycline at the nearest friendly base. If I can get enough, everyone gets, otherwise it's just my doctors' drug policy until I've got a steady supply.