I don't quite understand the saying when people say things like, "A cell is a cell is a cell". Why not just say, "A cell is a cell"?

They are pinging Gertrude Stein’s poem Sacred Emily (“Rose is a rose is a rose is a rose”). So your answer, I suppose, is for literary effect or emphasis and perhaps secondarily to show how artsy-fartsy they are.

Emphasis

It is as @perg said, it is a literary term.

For the same dumb reason they say the most important thing in real estate is location. But they say it three times. It’s so dumb I refuse to write it three times. I’d rather write all these extra characters instead.

Emphasis emphasis emphasis!

Also rhythm.

It may also be because groups of threes tend to stick in our minds.

I see, although it seems extraneous. Thank you : )

The repetition provides emphasis and implies never-endingness.

@perg is corrrect: it’s a play on Gertrude Stein’s famous line.

Because we like threes.
I’m being serious, mods!

To-morrow, and to-morrow, and to-morrow,
Creeps in this petty pace from day to day,
Macbeth A5, S5, L19–20
It’s cadence.

Once might be an accident, twice a coincidence, but three times establishes a pattern. We like some finality.