warmd controls caches used during startup and login. It is a launch daemon started by launchd during boot and is not meant to be invoked directly.
Apple forums describe it in more detail as:
While performing this work, warmd also listens for warming requests
from its associated launch
agent, warmd_agent. warmd will wait for such requests for a little
while after its launch to help the case
of a user booting to the login UI and login a little while later. The
maximum wait is 10min today.
warmd_agent is only launched during Aqua logins. It retrieves the
list of the 10 most recent
applications launched by the user, as reported by LaunchServices.