Let's say you launch Thunar and you see some file foo.abc does not launch any app when you double click on it. 

So you try to set association between the type of the file and some program -- a small window appears with available programs list.

The problem is, the list is not sorted, so finding out the right program is time consuming, because for example instead of jumping to "L" (for LibreOffice Writer) user has to linearly scan every entry to spot LibreOffice.