The strategy used to work the system may seem somewhat unnecessary at first, but there is a very specific process to it. The reason not to answer too many questions is that they're not only chances to gain compatibility points, but to lose them as well. Once you reach question no. 100, you've reached the maximum compatibility rating of 99%. Everything past that is a material difference, and there's no point in continuously answering questions solely for the sake of getting a 0.2% higher maximum compatibility when all it does is potentially lower your rating for people who would have very high ones normally.
Why should you never answer below "Somewhat important"? To put it bluntly, there's no point. A little important only gives you one higher point, which when stacked up to somewhat important's rating of 10 and very important's rating of 250 is pretty insignificant. If you have a slew of questions that you've marked as irrelevant or not very important clogging up your question database, it's only going to lower your match percentage over things that you really don't find to be that significant.
Lastly, the reason why you should go through your matches' profiles to find the questions they've most answered is to build a strong cluster of questions that will allow you to tweak your match percentages to your liking. While it's important to answer every question honestly, if you notice that there's someone who you like a lot yet have low compatibility with, you can go through the questions they care about and see if there are any more things you agree upon that can be used to raise your percentage to a satisfactory level.
Hopefully that sufficiently explains the reasoning behind the strategy outlined here!
O'Malley, H. (2014, January). How to Hack OKCupid. Paging Dr. Nerdlove. Retrieved from http://www.doctornerdlove.com/2014/01/how-to-hack-okcupid/