Why are websites constantly updated even when the change seemingly reduces functionality? Examples:


Imagine coming to SE tomorrow and instead of seeing "1 2 3 4 5 ... 77 next" at the bottom of the "questions" page, you see a giant circle with a triangle inside pointing to the right and you no longer have the option of going from page 1 to page 5 in one click, but you must now click the big circle incrementally instead.
Or imagine your favorite weather website having a 10-day forecast horizontally across the screen like a calendar would have, then one day it changes to a vertical alignment because someone decided it was time for a change.


Questions:

Why do people do this?  And is it normal to feel extremely frustrated by this?  I mean, when something works well and makes sense, leave it alone?.  Why change stuff just to change stuff?  And especially when the change is less functional or intuitive.
1)  Split testing designs in marketing (this is partially related to Jeromy's first point):  You don't know which type of layout or content will accomplish the ultimate goal you want your potential customers or users to attain for you.  Thus, you test several different varieties, measure the difference in them, factor in statistical validity, and get a better site/page for your goals.  These litte improvements can mean the difference betwee, campaign (lets say for example a paid one) making you a negative return on your investment(s) or could cause it to make you a million dollars this year.  When there are so many people online, the margins are thin and any little advantage you have will, essentially, be your profit most of the time.

2)  They hired someone who is "specialist at UI design" and are paying them on salary.  The UI is already complete, so they have all this free time on their hands to do nothing.  So instead of looking completely useless to their boss/company they start making arbitrary and unnecessary updates to the website/webpage many of which may seem pointless and skewed within actual testing that is going on.  They do this so they can keep their UI design jobs, that is, unless they're legitimately trying to test things and know how to go about doing this in the correct way (most have no clue what they're doing besides making something that is aesthetically pleasing and which could be obtained by simply looking at the UI's of other popular softare).

3)  Different theories of search engine optimization for your website:  A large theory out there is that Google likes sites with freshly updated content and that it is usually a positive variable towards increasing your rankings.  Part of the thing about SEO is is looking natural as possible, if you were to update too much content too frequently there's a possibility that their algorithm may sense a red flag.  Noone really knows how this works though. Thus some people may even prewrite content updates and then just release them as time goes on.