translatewiki.net there are many statistics. All of them serve a purpose, For MediaWiki purposes, the group statistics show how the many individual languages are doing. It provides inspiration to some translators as they compare their language to others.
A more meta view is provided by the group statistics in time. This is where the goals are found for the year. This month we exceeded our goal for the "most often used" messages. It is however unlikely that we will end the year in this way. The composition of this group of messages is scheduled to change and, this is likely to drop this number of languages considerably.
Changes to the "most often used messages" will have no effect on the other goals; if anything renewed interest in the most used messages will also increase the number of core messages that have been localised.
Such statistics have proven their worth; it is why similar statistics exist for the other projects. The best motivation is achieved when people notice that their work makes a difference and statistics are an important part of making that clear.