Standards compliance allows web developers to spend less time in QA and more time developing new features in THEIR applications. So rather than Microsoft developing one or two new features per year in their browser, Every web developer on the planet can develop one or two new features for their site per year. (Those numbers are obviously terrible and asspulled, but you get my meaning I'm sure).

It's similar to being able to write in higher level languages, (Java, Python) over lower level (C, Assembly). Once you don't have to care if the processor is x86 or Sparc, or if the compiler is GCC or MSVCC you can spend more time working on the actual purpose of your application. (Sorry, I couldn't think of a car analogy)

Remember all those #ifdef's in lots of old C (And many C++ Programs)?, ever had to write the same program twice in assembly, targetting two different processors? Ever written something once in python or java, and been reasonably confident that it'll run on any machine? (Java's stil a bit quirky between JVM Versions, but they're making a real effort at least), By standardizing the "language" (Or runtime environment in the case of most new languages), the productivity of every single person who uses that language improves.

That makes the investment of time by those writing the languages or runtime environments seem very worthwhile to me.