thread covers r basically aka cones. they cover the exposed threadings of say ego batts or ego type connectors when u use it with a slim type clearo/carto like ce2/boge. no particular function other than aesthetic purposes.
HOWEVER, if u do use a clearo (ce2), thread covers may protect the weak exposed base of the clearo. if u r using the cone-shaped clearo/carto, i.e. ce4/echo carto, u can't use a thread cover.