Because at bigger airfields, controllers often ask you to confirm it, even in a fixed gear aircraft, so the (admittedly pointless) addition to the call saves chatter.
The alternative is an unapproved "finals fixed gear" on the first circuit, or an even more unapproved "finals gear welded".
I have been asked to confirm my gear was down in a microlight at Culdrose and a motorglider at Boscombe Down, both obviously fixed gear.
G
[This message has been edited by Genghis the Engineer (edited 21 April 2001).]