Just like what I am asking in the title. I see nearly all the financial datas take logs before the data analysing step, Why? Dose it have nice properties?
Finance data tends to be money-related and as such incorporates many potential multiplicative effects - things like inflation or interest for example.
The variables tend to be right skew, in some cases close to lognormal.
The effects of things like scale changes (cents to dollars or billions to millions) are simple location shifts on the log scale. Multiplicative/percentage effects (like 10% increase, say) convert to shifts as well.
Variables often exhibit exponential growth, at least in the short term.
All these things make working on the log scale much more sensible/easy.