It'll confirm that the ticket holder has the right to be on that train, as not all Interrail/EUrail passes have exactly the same validity. And more importantly, it records the fact that the ticket holder was travelling on that date. Since some of the options are only valid for X days in Y, it's important that the number of travel days is recorded.
Any validity restrictions are already noted on the ticket itself - the diary is a completely generic form that looks the same for everybody until you start filling it in, so I still don't see how it helps "confirm that the ticket holder has the right to be on that train".
And likewise for the X days in Y-passes you already need to fill in the actual validity days
on the ticket itself before travelling and it says clearly that the ticket is the authoritative source in that regard.
but while they encourage you to send it in, it isn't compulsory so I don't think it is used directly to apportion the revenue.
Well, once you've finished travelling and are no longer under the threat of "your ticket isn't valid if you don't fill in the travel diary", there no longer is a good way to
force you sending in the travel diary anyway that wouldn't come across as rather unfriendly. Hence they only encourage you to do so and offer you a small reward if you actually manage to send it in...
And as for the revenue distribution, even if they're not apportioning each individual ticket according to its individual travel diary (if sent back), the revenue sharing is still likely to happen on the basis of how much people are travelling in each country on average, so they'd still want to have as many completed travel diaries as possible in order to increase the accuracy of those statistics.