The sand is used to create a heatsink. Its like a thermal capacitor. It absorbs and holds extra heat for when it is needed. Electric smokers are very slow at recovering heat loss. This comes from opening the door mainly. Most don't understand that electrics are not fire breathers.
Electrics need to remain closed as to not need to recover heat, buy doing so they do not require the addition of moisture because it not lost at the same rate. Firebreathers must be tended, therefore they must be opened (for the most part), thereby lossing heat and moisture which the heat is easily reclaimed and the mositure is added in the form of pans, spritz, mops, sauces.
I have read that the MES takes approx 15 mins to recoop the heat lost from opening the door and tending the meat. I think that is excessive, but lets say 10 mins.. So on a 12 hour smoke for a brisket, if you open the door only 1 time per hour to check or spritz or mop the meat, you have made a 12 hour smoke into a 14 hour smoke. Plus you've lost moisture.
The vent closed is completely up to you and how you use your smoker. You'll hear it must be open, I smoked on an analog smoker since the early 80's and it didn't have a vent, nor an air inlet. Most are more comfortible using the vent because that the way the firebreathers opperate and it works that way. There are long discussions here about vent position. Basically its all in how you want to use your smoker. There is no right or wrong way. You use what is most comfortable to you.
You can try it both ways and make your own decission. Me, I figure that the vent is adjustable for a reason, and I regulate it before, during and after the smoke. I want max. efficency of heat, minium electrial usage, as well as making both chips and pellets (for one of my aux. smoke generators), last as long as possible. Basically its a juggling act. It gives me something to do, so I leave the door closed and thats what its all about.
There is no wrong way, only the way you currently enjoy smoking.