[enriquto]

> But the authors writing down their best guess of the importance of their results helps others judge the minimum importance of the results, and decide whether they want to read the paper in the first place.

In many cases it will be unlikely that the authors even know the importance of the results. Forcing them to come up with an explanation would be useless at best, and an unbearable burden at worst (e.g., when a student has proved a theorem proposed by his advisor).

There are mathematical reviews, where mathematicians with a higher-level view on the field point to particular papers and explain their importance. Also, many journals have an editorial column that explains the papers on each issue.