Hi newby!
Do you mean those silicone end plugs with a hole in the middle? They're squishy - not like a hard plastic. They do serve a purpose or two. First, if you overfill and flood a carto they help to keep you from sucking in a mouth full of liquid. And to a lesser degree they impact how the air flow feels coming off the carto. But preventing drying out a carto? lol No, I don't think any of us ever put a pv down long enough for that to be a problem in need of a solution. (Seriously, e-liquid doesn't evaporate fast enough that even overnight parking without that cap would be a problem.)
Most of us just replace that silicone cap with a drip tip. Lot's of options there to fancy things up. And they come in all manner of shapes and materials from plastics to steel & aluminum to glass - with all manner of preferences for what feels best on the lips. And they look so much better than that silicone plug thingee.
Edit: Oh, you meant the top and bottom silicone covers that cartos usually come packed with? For storing and carrying exrta filled cartos around. Some people use the larger bottom cover to "condom fill" their cartos. But using one to cap your working carto when not in use? Really not necessary.
Welcome!