Why is a toilet flange on top of finished floor and not flush with it? (or at least I'm told)

I have seen them below, flush and set on the floor. The horn on the toilet will fit very tightly when the flange is on the floor. Flush is also OK. Should not be below the floor level or you can get leaks.

Wiser now,
There are several reasons for a toilet flange to be installed on top of the finished floor:

Most flanges are funnel shaped; this helps keep the bowl centered as the horn will contact the side of the flange before the bowl gets too far off center during its installation.
The bowl wax ( they are actually made of petroleum now ) will not be exposed to a probing snake, or toilet auger. When the horn is below the top of the flange, the gasket will be protected by the horn.
The bowl wax will not come in contact with water during, or after a flush. The horn protects the bowl wax from flow erosion when the horn extends below the top of the flange, in fact, a bowl gasket should only contact water when there is a back - up in the piping that the flange is connected to...

ma is correct but not bbelow the finished floor unless you double up the wax ring. The ideal height is top of flange flush with top of finished floor but you can go on top of the finished floor with not problems. The wax ring compresses up to 2" so eithet height is OK/

The "ideal height" is with the flange on TOP of the finished floor as Mike suggested. It is a "requirement" in all plumbing code books I own.

Flush with the floor works as well, of course, but plumbing code is very specific on this question!

Mark

... and in addition to all the responses above: from visual reasons, it "frames" the finished floor with over lapping ring.

Milo