Why is it necessary to drain water?

Are you talking about a recirculating hydroponics set up???If so plants absorb the nutrients and use them....there is an add back method you can use....and adding h202 to keep your res sterilized......so you don't have to change the res....but not changing the res can caUse all kinds of problems.....if the addback method is not done properly.....it is easier and safer to just change the reservoir every 10 -14 days

It is possible to have good results from never changing the res. It is more likely, without experience, that the yield will suffer.

Changing the res helps ensure the proper balance of the nutrient concentration. If the plants eat too much of one nutrient, it can cause others to get locked out. It is also possible for salts to build up inside the res and release later, again causing an imbalance in the concentration. Regular res changes and occasional flushes help prevent buildup and even washes away some of the old dead material. Also, plants will deposit stuff into the water from their roots; unwanted substances and even sugars to feed microbes. Changing the water also gives the roots a chance to sit in an air bath, witch usually results in a growth spurt the next day.

I don't like changing the res every 7 days because my nutes are expensive, but if I go past 10 days my plants start looking sad, but that could just be in my head. In the SeeMoreBuds video, the guy using the aquamist system changes his res only when he switches cycles, and only checks the ph once a week. So I suppose the situation and equipment has a lot to do with it.

"It is possible to have good results from never changing the res. It is more likely, without experience, that the yield will suffer
Changing the res helps ensure the proper balance of the nutrient concentration. If the plants eat too much of one nutrient, it can cause others to get locked out. It is also possible for salts to build up inside the res and release later, again causing an imbalance in the concentration".

I agree with Heisenberg statement above. i think its best to change the res every once in a while.
This is my first grow in dwc and im amazed by the ease at witch its going.
Only changed the res once after a month and a half of veg(two small plants 30litre dwc)
and that was at the start of 12/12. Now into week 4 of 12/12 and babys lookin real good not gonna
change res unless needed i do add oxy-plus every top up(add back) which might help.
As i dont have any experience i can only tell you what expirence i have had hope it helps.