What it means
Use it for long-term charts and assets with big percentage moves, like Bitcoin over years.
In practice
On a linear chart a move from 10 to 20 looks tiny next to a move from 1,000 to 1,100, although the first doubled and the second gained 10%. A log scale draws moves by percentage, so both are shown in true proportion.
Traders switch to it for charts that span years or assets that have multiplied many times. Trendlines and channels on such charts usually only make sense on a log scale.
What to watch for
A trendline drawn on one scale sits in a different place on the other. For intraday charts, where price changes by a few percent, the two scales look the same and linear is fine.