A lot of people do keep external lists of their purchases - some of us don't so it's great having it here as a reference whilst using the site and also provides a quick link to the various products & the SOTD feature.
If discussing something, it may have helpful notes about the particular version owned.
It's also useful having the lists here so that other people can see the sorts that you (or they) might like if you do any private sample swaps with other members or ask for recommendations, etc.
I've found it useful in reminding me to wear things that might otherwise be overlooked!
Where did that bottle go?...
Or reminding myself that something has actually been sampled, is on the next batch of samples to order, etc.
There must be other reasons, so others will probably remind us later.
There's a fun random 'pick me something to wear' feature - although it seems to have a tendency to suggest my least favourites...and it doesn't change it's mind if pressed twice!
The S.O.T.D. feature does keep track of the numbers of wears/last wear, although it's not up to date from Huddlerville as yet.