Ulti­ma­tely, the value of plan­ning isn’t that you execute the plan perfectly, that you catch every detail befo­re­hand, or that you predict the future; it’s that you enforce the self-disci­pline to think about the project in some depth before diving in and seeing what happens. […] The plan itself, howe­ver accu­rate it turns out, is less impor­tant than spen­ding time on the act of plan­ning. »

I totally agree. It is god damn hard to make accurate estimations about software development, because it is hard to know what will work or breaks and because the specifications are never complete until the full development