TBH, it was a very poor OP in terms of explanation - but not that surprising considering the poster's one and only (so far) other post, oh, and their apparently random made up user name.
The letter is primarily dealing with the field ministry, partly as that is the only thing 'officially' recorded - in unforgiving black and white - for all eternity on the publisher cards for all future other elders and CO's to view.
Therefore the 'importance' of the letter dealing with field ministry.
Meeting attendance is NOT recorded in the same way, at least not individually identifiably - but the letter stills highlights that elder's activity in other congregation activities (this would cover such things as meeting attendance, shepherding, elder's meetings etc etc) may be effected - and that other brothers can fill in for them.
Their caring for this important work may affect their field service activity as well as their share in other congregation activities. Elders in their congregation can show appreciation for brothers caring for other organizational needs by filling in for them locally when they must be away.