One thing it does is to delete the old routing data of each recorded/tracked AC in your "MyLog" Database. Result of doing this: MyLog Database gets smaller. ANRB Software starts faster. I'm not 100% sure if it's also affecting the "routing Tables" of the Navdata Database.
Remark: I noticed, that if I don't delete old data at least every three to four month the software sooner or later gets unstable and/or the start up takes ages.
On my 24/7 Netbook the "flush" interval is about 3-4Weeks.