The basis of it is that its a error in the coding which can cause your entire sheet/page to arse up from that point on. A lot of browsers these days can swallow some of these mistakes and therefore sites like amazon can get away with 1463 errors. But it may risk your site being unviewable if you error is more fatal.
Partly it's out of perfection and love of the craft, partly out of reducing any issues further down the line.
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