lol.. there is no filter in ecigs - no tar to catch ROFL!
The cartos have filler which holds some ejuice so you aren't having to constantly add juice. Back in the old days we had atomizers with cartridges that were basically a teeny, tiny sponge to hold a few drops. Then drip tips came along and some of us ditched the cartridges and went to direct dripping. You drip a few drops, you vape it...then repeat. There is no filler to hold extra juice, just what the wick holds.
I would not recommend dripping and driving though lol. If you drip juice on an atty and vape you get a flavor that is unmarred by fillers, etc. Truly a great flavor experience in my opinion but... I understand most people want to vape without needing to add juice constantly. I'm just spoiled by dripping so I'm still doing it though cartos have much improved and now there are a gazillion tanks on the market. Something for everyone's needs/wants I think.