Until they go into their cocoons it’s all about fattening up.
I am always amazed that all I have to do is brush against a milkweed leaf and it will exude its milky liquid while the caterpillar can chow down on the same leaf and leave no trail of sap. And that toxic fluid is what gives them their protection against predators. Monarchs-Danaus plexippus have built up a tolerance for the latex where it can sicken or kill others. But the milkweed doesn’t wish to be eaten so is constantly evolving new deterrents. It’s an ongoing co-evolutionary battle. If you are interested in learning more about the subject, “Monarchs and MIlkweed” is a good source.
A hungry larva can devour a leaf fairly quickly.
I haven’t found any milkweed tussocks-Euchaetes egle so far this year, but they also can enjoy the leaves or pods without any harmful effects.